Data Formats and Their Sugggested File Extensions
An extensive listing of file name extensions with brief description.
www.uni-koeln.de Details

DaubNET File Format Collection
The full specifications of various file formats.
www.daubnet.com Details

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

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.
www.stack.com Details

Webopedia - Data Formats
Contains information about many data formats.
webopedia.com Details

Wotsit's Format
Detailed file formats and data formats for programmers. A large collection of programming resources with detailed information.
www.wotsit.org Details