Colnect (many languages) – Colnect (premium) has auto-matching, best matches and an extensive free stamp catalogs
Michel Online Catalogue (German/English/Italian) (from € 177.24/year, discounts available)
Stanley Gibbons (English) (£ 39,95/year)
Yvert&Tellier (French) (€ 129.00/year for global, cheaper subscriptions available)
LastDodo (English/German/Dutch/French) – Wiki based free catalogue to store your collections. The catalogue contains references to Michel/Scott/Stanley Gibbons/Yvert and gives you the opportunity to sell your stamps in your own shop or sell it at auction.
Freestampcatalogue (English/German/French/Spanish/Dutch) – All official stamps since 1920 (with reference to Michel/Scott/Stanley Gibbons/Yvert) and for many countries pre-1920 stamps.
Francophone Africa Stamps (French/English) – Over 30,000 stamps of African nations (with Yvert reference)
GPFS (English/French/Danish) – Stamps from France and Scandinavia
Magan Stamps (English) – Stamps from Middle East Arabia (with references to Michel/Scott/SG)
Neofila (English) – East Europe & ex-USSR, Africa and Korea (North)
New Stamps of the World – Nice site on new stamp issues worldwide
Perf48 (English) – Worldwide stamp database. Membership to take a data-driven approach to stamp collecting and track your collection online (USD 19.48)
Stampdata (English) – Wiki based free catalogue; stamps of 960 countries and areas and over 205,000 stamps.
StampWorld (Various) – The Facebook of Stamp Collectors; over 650,000 stamps.
WNS (English/French) – WADP Numbering System (WNS) with stamps from all participating countries since 2002
Stamps of the World – Wiki based free catalogue; almost 114,000 pages
Åland
Posten Åland (Swedish/Finnish/English/German) – Stamps since 1984
Postimerkkiluettelo Suomi (Finnish) – Stamps till 2013
Albania
Posta Shqiptare (Albanian/English) – Stamps since 2010
Alderney
WOPA (English/German/Spanish) – World Online Philatelic Agency: Stamps since 2006
Andorra
CEREOS (Spanish/English/French/Galician/Catalan/Basque/German) – All stamps since 1997
CÉRÈS Philatélie (French) – Official catalogue with stamps since 1966
Philatelie.fr (French/English/German) – All stamps of French Andorra, with Yvert catalogue numbers
Anguilla
Anguilla Postal Service (English) – Stamps since 1986 (not complete)
Armenia
Armenian Stamps (English) – Stamps since 1992 with Scott reference
Ukrafil (English/French/German/Russian) – Stamps since 1992
Australia
Australian On-line stamp catalogue (English) – All stamps
PreDecimal Stamps of Australia (English) – Stamps from 1901 to 1965
Austria
Austrian Post (German/English) – Stamps since 1947
Gerhard’s Briefmarkenseite (English) – All stamps since 2000 with Michel reference
StampsAustria (German/English) – All stamps until 1985, with Michel and Scott numbers
Azerbaijan
Ukrafil (English/French/German/Russian) – Stamps since 1992
Belarus
FSU Stamps (English) – Stamps since 1992 with Michel numbering and mint and used prices
Belpochta (English/Belarusian/Russian) – All stamps since 1992
Ukrafil (English/French/German/Russian) – Stamps since 1992
Belgium
Belgium Stamps (English) – All stamps
StampsBelgium (French/English) – All stamps until 1990, with Yvert numbers
Bolivia
Filatia de Bolivia (Spanish) – All stamps
Bosnia and Herzegovina
BH Posta (Bosnian/English) – All stamps since 1995
Hrvatska posta d.o.o. Mostar (Croatian) – All stamps since 1993
Brazil
Abrafite (Portuguese) – RHM, Yvert, Scott and Michel catalogue numbers
Catálogo de Sêlos de Brasil (Portuguese) – with price indication
Selo Show (Portuguese) – All stamps since 1900.
StampsBrazil (Portuguese/English) – Stamps till 2000, with RHM and Scott catalogue numbers
Bulgaria
Stamps of Bulgaria (English) – Stamps since 1990
Republic of Bulgaria (Bulgarian) – Stamps from 1879 till 2005
Canada
Stamps and Canada (English) – All stamps until 1999, with Scott numbers and valuation
Vista (English) – Complete free catalogue of Canada
Chile
Chile Collector (Spanish/English) – All stamps since 1853
China
Stamps-China (English) – All stamps until 1980, with Scott numbers
Xabusiness (English) – All stamps since 1990 with Scott reference
Croatia
Hrvatska pošta (English/Croatian) – All stamps since 1991
Czech Republic
Filaso (Czech) – All stamps from the Czech Republic
Internetová Obchodní Filatelie (Czech/English) – All stamps from the Czech Republic and former Czechoslovakia, including Michel and Pofis numbers
Penkava (Czech) – katalog známek Česka republika
Svet Filatelie (Czech/English) – All stamps from the Czech Republic
Czechoslovakia
Filaso (Czech) – All stamps from Czechoslovakia (1945-1992)
Denmark
Post & Tele Museum Danmark (Danish/English/Chinese/French/German) – Complete free catalogue of Denmark with numbering
Perfinsamlere (all languages) – Online catalogue of Danish perfins
Frimindesk (Danish/English) – Complete free catalogue of Denmark till 2011
Egypt
Egypt (English) – Stamps of Egypt/UAR from 1866 until 1963, including Scott and Stanley Gibbons catalogue numbers
Estonia
Eesti Post (Estonian/English/Russian)– Stamps since 1992
FSU Stamps (English) – Stamps since 1992 with Michel numbering and mint and used prices
Ukrafil (English/French/German/Russian) – Stamps since 1991
Faroe Islands
Posta Faroe Islands (Faroese/Danish/German/English/French) – Stamps since 1979
Finland
Postimerkkiluettelo Suomi (Finnish) – All stamps till 2010
France
CÉRÈS Philatélie (French) – Official French catalogue
Philatelie Free (English/French/German/Spanish) – Free catalogue of France
Philatelie.fr (French/English/German) – All French stamps, with Yvert catalogue numbers
StampsFrance (French/English) – All stamps till 1990, with Yvert and Scott catalogue numbers and prices
Timbres de France (French) – All French stamps, with Yvert catalogue numbers
French Polynesia
CÉRÈS Philatélie (French) – Official catalogue with stamps since 1962
French Southern and Antartic Lands
CÉRÈS Philatélie (French) – Official catalogue with stamps since 1969
Georgia
Ukrafil (English/French/German/Russian) – Stamps since 1992
Germany
Frimindeks (English) – Deutsches Reich 1872 – 1945
Gerhard’s Briefmarkenseite (English) – All stamps since 2000 with Michel reference
German Stamps.net (German/English) – Catalogue of stamps till 1945
Stamps-German (German) – All stamps until 1945 with Michel and Scott numbers
Gibraltar
Gibraltar Stamps (English/Spanish/French/Italian/German) – Stamps since 1995
WOPA (English/German/Spanish) – World Online Philatelic Agency: Stamps since 2000
Greenland
Frimindeks (English) – All stamps of 1938 – 2006
WOPA (English/German/Spanish) – World Online Philatelic Agency: Stamps since 2007
Guernsey
WOPA (English/German/Spanish) – World Online Philatelic Agency: Stamps since 2006
Heligoland
Heligoland Stamps (English) – Extensive website on Helgoland stamps
Hong Kong
Hongkong Post (English/Chinese) – Stamps since 2014
Hungary
Magyar Posta (Hungarian/English) – Stamps since 2012
Iceland
Frimindeks (English) – All stamps of 1873 – 1947
Graungaard (English) – Specialized stamp catalogue of Iceland, all stamps till 1944
Ícelandspóstur (Icelandic/Danish/English/German/French) – Stamps of Iceland since 1873 in pdf
India
Indian Postage Stamps (English) – All stamps till 1991, with Scott and Stanley Gibbons catalogue numbers
Indonesia
Eldi Philately World (English) – Indonesian stamps of 1996 – 2003
Isle of Man
WOPA (English/German/Spanish) – World Online Philatelic Agency: Stamps since 2010
Israel
Israel Philatelic Federation (English/Israelian) – All stamps of Israel
WOPA (English/German/Spanish) – World Online Philatelic Agency: Stamps since 2009
Italy
iBolli (Italian) – Complete free catalogue of Italy and European stamps
i bolli delle colonie (Italian) – Italian colonies
Stamps of Italy and Italian Colonies (English) – with Sassone catalogue numbers
StampsItaly (Italian/English) – All stamps till 1990, with Sassone and Scott catalogue numbers
Jersey
WOPA (English/German/Spanish) – World Online Philatelic Agency: Stamps since 2010
Jordan
Stamps of the Hashemite Kingdom of Jordan (English) – All stamps from Jordan with Scott and Stanley Gibbons catalogue numbers
Kazakhstan
Kazpost (English/Kazakh/Russian) – Stamps since 1992
Ukrafil (English/French/German/Russian) – Stamps since 1992
Kosovo
Posta Kosoves (English/Albanese/Serbian) – All stamps since 2000
Kyrgyzstan
Ukrafil (English/French/German/Russian) – Stamps since 1992
Latvia
Former Soviet Union (English) – Stamps since 1991 with Michel catalogue numbers and mint and used prices
Latvijas Pasts (Latvian) – All stamps since 1991 with extended description
Ukrafil (English/French/German/Russian) – Stamps since 1991
Liechtenstein
Gerhards Briefmarkenseit (German) – Stamps since 2000
Lithuania
Former Soviet Union (English) – Stamps since 1990 with Michel catalogue numbers and mint and used prices
Lithuania (Lithuanian) – All stamps from 1918 until 1940, with Michel catalogue numbers
Lithuania (Lithuanian) – All stamps from 1918 until 1940, with Scott and Michel catalogue numbers
Ukrafil (English/French/German/Russian) – Stamps since 1990
Luxembourg
Luxembourg Stamp Database (English) – All stamps till 2000
Malta
Maltapost (English) – Excellent complete free catalogue for Malta
Moldova
Moldova Stamps (English) – All stamps since 1991
Posta Moldovei (English/Romanian/Russian) – All stamps since 1991
Former Soviet Union (English) – Stamps since 1991 with Michel catalogue numbers and mint and used prices
Ukrafil (English/French/German/Russian) – Stamps since 1991
Monaco
CÉRÈS Philatélie (French) – Official catalogue with stamps since 1960
Philatelie.fr (French/English/German) – All stamps of Monaco, with Yvert catalogue numbers
Timbres de France (French) – All stamps of Monaco with Yvert catalogue numbers
Timbres Monaco (French) – Free catalogue of Monaco
Montenegro
Posta Crne Gore (Croatian) – All stamps since 2005
Namibia
Nampost Philately (English) – All stamps since 2000
New Caledonia
CÉRÈS Philatélie (French) – Official catalogue with stamps since 1962
New Hebrides
Virtual Stamp Album New Hebrides (English) – Complete free catalogue
New Zealand
Auckland City Stamps (English) – Official catalogue online
New Zealand Post (English) – All New Zealand stamps
Stamps NZ (English) – All New Zealand stamps
Niue
New Zealand Post (English) – All stamps since 2009
Norfolk Island
Norfolk Island Stamps (English) – All stamps since 2000
Northern Cyprus
Yildirim Philately (English) – All stamps since 1973
Norway
Frimerkehuset (Norwegian/English) – All stamps of Norway
Pakistan
Pakistan Philately (English) – Siddiqi catalogue with all stamps since 1947
Pakistan Post (English) – All stamps since 2001
Papua New Guinea
Post PNG (English) – All stamps since 2011
Peru
Stamps of Peru (Spanish/English) – All stamps of Peru wit Scott reference
Poland
Filatelistyka (Polish) – All stamps till 2006
KZP (Polish) – All stamps
Stamp Encyclopaedia Poland (English) – The Postage Stamps of Poland
Portugal
StampsPortugal (Portuguese/English) – Stamps till 2012, with Afinsa and Scott catalogue numbers
WOPA (English/German/Spanish) – World Online Philatelic Agency: Stamps since 2006
Romania
Romaniastamps (English) – All stamps since 1990
Russia
Marka-Art (English/Russian) – Stamps since 1992
Stamprus (Russian) – All stamps since 1857, including USSR and mint and canceled prices
Ukrafil (English/French/German/Russian) – Stamps since 1992
Saint Pierre and Miquelon
CÉRÈS Philatélie (French) – Official catalogue with stamps since 1962
San Marino
iBolli (Italian) – Complete free catalogue of San Marino
Schleswig-Holstein
Frimindeks (English) – All stamps of 1850 – 1920
Singapore
CS Philatelic Agency (English) – Stamps since 1948 (Singapore) and 1861-1941 (Strait Settlements)
Slovakia
Pofis (Slovak/English) – All Slovak stamps since 1993
Známková Tvorba (Slovak) – All Slovak stamps since 1993
Slovenia
Maxstamps (English/German) – All stamps since 1991
Soviet Union
Stamp Russia (English) – All stamps from 1918-1990 (R.S.F.S.R and USSR period)
Spain
Catalogo de Sellos (Spanish) – All Spanish stamps
Sweden
Schedische Briefmarken (German) – Overview of all Swedish stamps with some background information. No catalogue numbers
Switzerland
Gerhard’s Briefmarkenseite (English) – All stamps since 2000 with Michel reference
Swiss Stamps (English) – Swiss stamps of 1940 – 2009
Tajikistan
Ukrafil (English/French/German/Russian) – Stamps since 1992
Thailand
Thai Stamps (English) – Complete free catalogue of Thailand
Tokelau
New Zealand Post (English) – All stamps since 2011
Tunisia
Tunisian Postal Stamps (English) – Complete free catalogue of Tunesia
Turkey
Yildirim Philately (English) – All Turkish stamps since 1921
Turkmenistan
Ukrafil (English/French/German/Russian) – Stamps since 1992
Tuvalu Islands
Stamps of Tuvalu (English) – All stamps since 1976
Ukraine
Simon’s Stamp Exchanging Page (English) – Ukrainian stamps 1992 – 2005, with reference numbers of all major catalogue publishers
Ukrafil(English/French/German/Russian) – Stamps since 1992
United Kingdom
British Stamp Catalogue (English) – All stamps till 2008 (not very user friendly)
Victorian Stamps (English) – Great Britain Victorian Stamps (till 1970)
United Nations
UNOstamps (English) – United Nations related stamps, with Yvert, Michel and Stanley Gibbons catalogue numbers
United States
Stamp Catalogue (English) – Complete free catalogue of the USA
Vista (English) – Complete free catalogue of the USA
Uzbekistan
Ukrafil (English/French/German/Russian) – Stamps since 1992
Vatican
iBolli (Italian) – Complete free catalogue of the Vatican
Vietnam
Vietnam Stamp (English/Vietnamese) – All Vietnamese stamps
Vietstamps (English/Vietnames) – All Vietnamese stamps since 1945
Wallis and Futuna
CÉRÈS Philatélie (French) – Official catalogue with stamps since 1962