Wotsit's Format

Detailed file formats and data formats for programmers. A large collection of programming resources with detailed information.
http://www.wotsit.org/

DaubNET File Format Collection

The full specifications of various file formats.
http://www.daubnet.com/formats/

Almost Every File Format in the World

Names many file formats and their extensions.
http://www.ace.net.nz/tech/TechFileFormat.html

Data Formats and Their Sugggested File Extensions

An extensive listing of file name extensions with brief description.
http://www.uni-koeln.de/themen/Graphik/ImageProcessing/fileext.html

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.
http://www.stack.com/file/extension/index.html

File Extensions

Simple alphabetical listing of file extensions and descriptions, and in many cases, the name of the associated program.
http://iar.levillage.org/utilities/dataextensions.html

Webopedia - Data Formats

Contains information about many data formats.
http://webopedia.com/Data/Data_Formats/