File is the open source implementation of the file command used on almost every free operating system (OpenBSD, Linux, FreeBSD, NetBSD) and also on systems that use free software (including OS/2, DOS, MS Windows, etc.).
The file command, if you're not familiar with it, is a command-line tool that tells you in words what kind of data a file contains. Unlike MS-Windows, UNIX and other systems don't rely on filename extentions to tell you the type of a file, but look at the file's actual contents. This is, of course, more reliable, but requires a bit of I/O.
The original file command shipped with Bell Labs UNIX but was unavailable in source form to the masses before Ian's reimplementation.
This file command (and magic file) was originally written by Ian Darwin (who still contributes occasionally) and is now maintained by a group of developers lead by Christos Zoulas.
Who's using it?
Every known BSD distribution (FreeBSD, NetBSD, OpenBSD, Darwin/Mac OS X, etc)
Every known Linux distribution
The Apache httpd server mod_mime_magic module uses the file command's innards to make file type guessing more reliable under Apache HTTPD.
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