Published and Online Thematic catalogues
Various themes
phildom philately (English/Spanish) - Domfil Philately. Lots
of thematic catagories, with Scott and Yvert numbers. For online catalogue a buy in the shop is required
Freestampcatalogue
(English/German/French/Spanish/Dutch) - Thematic search (with reference to Michel/Scott/SG/Yvert) 
Neofila (English) - Birds, Cats, Flora and WWF
Aircraft
Stanley Gibbons (English) - Collect
Aircraft on Stamps
Anti-Smoking
Anti-Smoking Stamps (English)
Astro
Weebau
(English) - Online catalogue of astro stamps (no images),
including catalogue numbers of all major catalogue publishers
Bears
Bearstamps
(English) - including catalogue numbers of all major catalogue
publishers
Bees
Api-Phila (German/English) - The World of Bees on Stamps
Beetles
Beetles on Stamp (English/French)
Bible
The Bible Odyssey on Israeli Stamps (English)
Birds
Birds of the
World on Postage Stamps (English) - All birds on stamps per
country. No reference details
Europas
fugle (Danish) - Printed catalogue
Maudoc
(Italian/English) - Stamps of Italian area featuring birds
Michel (German) - Motivkatalog Vögel
Europa
mit CD
Theme Birds on stamps (English) - Complete
list of birdstamps worldwide, including catalogue numbers of all major
catalogue publishers
Butterfly
Butterflies
(Russian) - including catalogue numbers of all major catalogue
publishers
Cats
CAT a Logo (Italian) - Cats on stamps with Yvert catalogue numbers
Cats on Stamps (English) - Cats on stamps with Yvert and Michel catalogue numbers
Chess
Chess on Stamps (English) - Chess on stamps worldwide compete till 1990, including catalogue numbers of all major catalogue publishers
Cricket
Nazim's Cricket Page (English)
Countries
LituanicaOnStamps (English/Lithuanian) - Lithuania on World Postage Stamps
Detective Fiction
Detective Fiction on Stamps (English) - Worldwide stamps, with catalogue numbers of all major catalogue publishers
Ducks
The Federal Duck Stamp Program (Englsih) - Ducks on US stamps with detailed information
Entomology
Entomophila (English/French) - Worldwide stamps, with Scott and Yvert catalogue numbers
Europe
iBolli (Italian) - all European stamps
Michel (German) - CEPT Katalog
Fairy tales
Hans Christian Andersen (English) - with catalogue numbers of all major catalogue publishers
Fauna
Fauna
Stamps (English/Russian)
Fish
Exotic Fish (German)
Giant Panda
Giant Panda Stamps (English) - with Scott catalogue number
Golf
Golf on Stamps (English/French) -
including catalogue numbers of all major catalogue publishers
Horses
Horse
on Stamps (English/Russian) - Horses on stamps
Lighthouses
Lighthouses on Stamps (English) - Lighthouses on stamps worldwide
Machin
Great Brittain Machin (English) - Not a real catalogue, but with lots of information and album pages
Machin Stamps (English) - The Connoisseur Catalogue of Machin Stamps and Decimal Definitives
Maps
Maps on stamps (English) - with Scott, Michel and Stanley Gibbons catalogue numbers
Mathematicians
Mathematical Stamp Collecting (English)
Mathematicians on Stamps (English)
Meteorites
Meteorites on stamps (English) - with Scott catalogue numbers
Metro/Underground
MetroPhila (German) - Metro on stamps. Now Germany, started with Europe and Worldwide will follow in the future
Money
Coins on Stamps
(English/German/Hungarian) - Money on stamps collection, with
catalogue numbers
Motor Vehicles
Stanley Gibbons (English) - Collect Motor
Vehicles on Stamps
Olympics
Olympic Stamps Catalogue (English/Spanish) - Online and Published stamp catalogue
Paintings
Arago (English) - American Art on Postage Stamps (Smithsonian National Postal Museum)
Art on Stamps (English/German/French/Romanian)
Hundertwasser Stamps (English) - Very nice site on the painter Hundertwasser
Paintings on Stamps (English) - Painting on stamps all over the world. Only pictures, no references
The Stamp Gallery of Czech and Slovak Graphic Art (German/English)
People
Ghandi (English) - Stamps related to Moandas K. Ghandi
Ghandi (English) - Ghandistamps with stamp about Ghandi worldwide
Gandhi (English)
Jules Verne (English) - Jules Verne Stamps
Plants
Plants on
Post Stamps (Russian/English)
Paleontology
Dinosaurs on stamps (English) - with Scott reference
Paleontology (English) - including catalogue numbers of all major catalogue publishers
Paleophilatelie (English) - Very extensive overview of stamp related to paleontology
The
Illustrated Catalogue of Dinosaur Stamps (English)
Railways
Norwegian Railways in Philately (English)
Reptiles
Reptile Stamps (English) - with Michel catalogue numbers
Roses
Roses on Postage Stamps (English) - including catalogue numbers of all major catalogue publishers
Ships
Michel (German) - Motivkatalog
Schiffe ganze Welf auf CD
My Watercraft Philately (English)
Trains
Michel (German) - Motivkatalog Eisenbahnen
ganze Welt mit CD
Turtles and Tortoises
Donald N. Riemer (English) - A worldwide list of turtles and tortoises on stamps (including Scott reference)
Sea Turtle Postage Stamps (English) - Sea turtle postage stamps of the world, including catalogue numbers of all major catalogue publishers
Unusual
Dentelli
Stravaganti (Italian/English) - Unusual and unconventional
stamps all around the world
Volcano
Iomoon (English) - Volcanoes on stamps worldwide
Women
Famous Women on Stamps (English) - Famous women on stamps from Norway and Finland
WWF
Groth AG (German) - WWF Briefmarken Katalog
WWF
Stamp Catalogue (English/Russian) - WWF stamps since 1987