Freshwater Aquariums
An About.com site authored by Shirlie Sharpe, lots of information and net links on freshwater fish/aquariums.

Saltwater Aquariums
An About.com site authored by Stan & Debbie Hauter, devoted solely to the salt water aquarium enthusiast, with lots of great information and resources.

AquaLink
This information service provides a huge resource to aquarium enthusiasts of all kinds - includes articles, forums, services, community and shopping.

Aquaria FAQs
A huge collection of FAQs (various authors) covering aquarium set-up, fish, plants, diseases, breeding, filtration and food.

FishGeeks.com
Lots of information here including articles, message boards, FAQs, a bookstore and aquarium products, and other miscellaneous useful and fun pages.

Fish Information Service
Many archived articles (assembled by Mark Rosenstein) about aquariums, covering freshwater, marine, tropical and temperate.  Good resource.

Freshwater Fish FAQs
FAQ, and everything from fish bowls to long term aquarium planning, as well as plants, algae, medical stuff, and breeding information.  The Author will answer questions on line as well.