Data Formats and Their Sugggested File Extensions
An extensive listing of file name extensions with brief description.

DaubNET File Format Collection
The full specifications of various file formats.

FileInfo.net
Database of file extensions with information about each file type.

Various File Formats 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.