Dot What!?
Database of file format extensions, mainly for Windows, but also Apple Macintosh and Linux.
File formats and their Extensions
Large list with thousands of file formats.
File-Extensions.org
A search engine and large list of file extensions. Each file extension comes with an explanation on the way it is used today.
FileExt
Search engine for file extensions. It will find the type of file a file-extension represents and/or the program using that file type.
The Graphics file formats page
Graphics file formats, programming code, FAQs, etc.
Webopedia
Data formats and their file extensions.
Wotsit
Information on hundreds of different file types and all sorts of other useful programming information; algorithms, source code, specifications, etc.