Colin Rose has assembled a chess stamp exhibit as well as a chess stamp catalog that is organized according to date, country, and theme.
Jacques Foissotte also maintains an online catalog that also features some autographs and covers.
Steve Trussel presents a nice exhibit of early chess stamp issues as well as a chess on stamps checklist.
Conrad Goodman is assembling a comprehensive online catalog of all stamps and varieties with estimated prices
Lutz Seger is building the largest online chess stamp catalog. In addition to images of stamps and many covers, the site contains useful information about related tournaments and chess events.
Ricardo Andreis has assembled a comprehensive catalog of chess stamps and postmarks after 1983. Don't miss his pages on the best items in his collection and his wonderful exhibit of chess postmarks from Italy.
Gerarth Radosztik has assembled an illustrated online catalog of chess postcards and he has just added a new section on chess autographs!
Nico van der Plas runs a periodic auction of chess stamps and has another chess stamp site with much useful information (in Dutch though).
Once you are interested in collecting chess stamps, be sure to check out the Chess on Stamps Study Unit. They have a quarterly publication, a sales unit, and a regular auction to help build up your collection.
And the German Association GSM (Gemeinschaft der Schachmotivsammler) has a web site and publication.
Vladislav Amigood maintains an interesting exhibition of Russian chess stamps.
Eddy Bruns keeps a nice exhibit of the chess stamps and postmarks of Belgium.
Alberto Cabral de Mello Borges has begun a comprehensive look at his chess stamp collection. Check back often... his is a large and impressive effort!