The Fishbase is a web-accessible database of fish information. This may sound somewhat... dull... until you actually start playing with it. With over 28,000 fish listed (including my favorite, the Coelacanth), the Fishbase lets you explore by environmental information, location, biological data (including genetic records), class, even how dangerous a given fish may be and whether it appears on a stamp somewhere in the world. The volume of material on fish species is an object lesson in biodiversity.
The site contains more than the database. There's an online course in icthyology, links to other resources such as LarvalBase (a database of fish larvae identification), even a discussion forum for all of your pressing fish-related queries. And if you need access to the Fishbase while away from the Internet, you can purchase the entire database for a nominal fee. This is just the kind of site that makes the web great.
Cool stuff, there is also (not quite as cool, no courses, or forums ) a similar database for birds http://www.bsc-eoc.org/avibase/avibase.jsp?pg=home&lang=EN