Hexbrowser is a tool that can identify more than 1000 different file formats, by looking for signatures inside the files.

HexBrowser is a tool that identifies file types. It does not care about the file extension, but opens each file to look for signatures inside, so it can determine the exact type of a file. It now recognizes more than 1000 different file formats. HexBrowser shows detailed information about each file, or a hex- or text dump of the beginning of each file. For the sake of speed, only the first 12 KB of the file is shown.

Here are some of the file formats that it recognizes:

Audio:

real, wav, mp3, decaudio, adlib, sunraster, adif, midi, avr, au, vqf, alp, esps
Video:

real, asf/wmv, mpg, mov, 3gp, mp4, mjpeg, dv, flac, nsv, mng, swf, mtv, fli, flc, bik
Graphics:

jpg, jpeg2000, bmp, gif, wmf, tiff, openexr, mdi, kdf, viff, psd, mrsid, cmf, png, jng, wpg, psp, iff, emf, eps, ps, pcx, pic
Office:

MSOffice, OpenOffice, PocketWord, StarOffice, Write, iWorks, txt, rtf, html, pdf, chm, pst, xml
Archives:

zip, rar, lha, larc, installshield, c64, sbx, daa, cab, tzf, jar, sit, gzip, ghost, zoo, hyp, ima, arj, 7-zip, z, lzx, hqx, deb, rpm
Other:

mac/linux/unix/dos/windows/amiga/ps3 executables, database files, fonts+++

Limitations:
Requires Microsoft .NET Framework