Apistogramma

Apistogramma.com
The best forum worldwide if you are interested in Apistogramma and other dwarf cichlids.

BARSCH - Die Bremer Aquarienrunde für Südamerikanische-Cichliden und Harnischwelse
A lot of great info about Apistogramma. It also lists most of the tradenames, and connects them to the right species

Cichlids:

Alf Stalsbergs homepage
Alf's homepage is well worth a visit. He has accumulated a tremendous amount of knowledge over the years,
and is one of the profiles in the hobby, at least in Scandinavia and Europe.
Make sure you read the fascinating stories from his collection trips throughout the Americas as well as Africa.

Catfish
Planet catfish
"The catfish is a plenty good enough fish for anyone"

Corydoras  world
Probably the best corydoras site on the web. 15£  to pay to become a memeber.

Danio
Danios and Devarios
Excellent Danio page