Almost Every File Format in the World: Names many file formats and their extensions.
DaubNET File Format Collection: The full specifications of various file formats.
FileInfo.net: Database of file extensions with information about each file type.
FILExt: Searchable database of file extensions for data formats, with information and notes on each.
Various File Formats and How to Deal With Them: Includes descriptions and suggestions for software (free and pay) that handle them on Windows and MacOS.
Webopedia - Data Formats: Contains information about many data formats.
Wotsit's Format: Detailed file formats and data formats for programmers. A large collection of programming resources with detailed information.