Corydoras.net, general informaion

Planet Catfish, one of the best resource on the net for everything catfish.
ScotCat, another very good site.
Corydoras World, Ian Fuller's superb site about Cory catfish. Probably the person in the world who has bred the most species of Corydoras.
Corydoras Encyclopedia, a site with an enormous amount of photos.

General sites with catfish information

Master Index of Freshwater Fishes, Over 40,000 references to the over 60 aquarium books and selected issues of 6 magazines.

Articles

Cory Corner by Paul Schuman (Aqualink Column) (1996-98)
Cory Corner Web Sites and Unusual Corys, by Paul Schuman (Note that Paul's own site is shut down!) (Aqualink Column) (June 97)
Crazy About Corydoras, by Paul Schuman
Corydoras Resources in Print (Fish Link Central) (March 1997)
Those Collectable Corydoras - Part I, by Gian Padovani (Aqualink Column) (May 1997)
Those Collectable Corydoras - Part II, by Gian Padovani (Aqualink Column) (June 1997)
Barbels and Scientific Names, by Gian Padovani (Aqualink Column) (April 1997)

Catfish Associations

Guardians of Catfish, info about this Scandinavian Society (in Swedish)
Catfish Study Group UK, info about this society

Please, let me know of more links.