Data Formats

Almost Every File Format in the World - Names many file formats and their extensions.
http://www.ace.net.nz/tech/TechFileFormat.html

File-Extensions.org - Searchable file extension list including descriptions and associated applications.
http://www.file-extensions.org/

FileSuffix.com - A searchable filename extension database.
http://www.filesuffix.com/

Macintosh Type/Creator Database - A database of windows file extension codes with correlating Macintosh Type/Creator code pairs.
http://homepage.mac.com/tcdb/

Dot What?! - Database of file extension information. Supports users comments and submissions. Includes discussion forum.
http://dotwhat.net/

File Formats Magic Database - Contains information on main file formats as well as an FFID registry and a signature file permitting the identification of files from their contents. Also contains tips on how to design new file formats.
http://www.magicdb.org/

DaubNET File Format Collection - The full specifications of various file formats.
http://www.daubnet.com/en/file-formats

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

Sustainability of Digital Formats Planning for Library of Congress Collections - Provides information about digital content formats from the viewpoint of maintaining sustainable and accessible archives.
http://www.digitalpreservation.gov/formats/

FILExt - Searchable database of file extensions for data formats, with information and notes on each.
http://filext.com/

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

UK National Archives PRONOM Technical Registry - Resource for those requiring definitive information about the file formats, software products and other technical components required to support long-term access to electronic records.
http://www.nationalarchives.gov.uk/PRONOM/

Filerecon Project - Developing a program to automatically identify files from their content. Project news, FAQ, and download. [GPL]
http://www.computec.ch/projekte/filerecon/

FileInfo.com - Database of file extensions with information about each file type.
http://www.fileinfo.com/

Uniblue File Extension - Searchable database of file extensions, explaining what they are used for and which programs they are associated with.
http://www.file-extension.com/