Windows Essentials Codec Pack
If you've ever come across a media file that wouldn't play in your media player, this freebie is the answer to your needs. It installs the codecs--short for coders-decoders--that your media player needs in order to play back many different kinds of media.
This isn't actually a standalone program. Instead, it gives your media player access to the codecs. So just install it, and you'll be able to view or listen to a wide variety of media files with your existing media player. And if you're not happy with your existing media player, it includes a copy of Media Player Classic, a simple, stripped-down media player that looks and works much like the first versions of Windows Media Player.
For the techies out there, here's the list of what it handles: Audio CDs, DVDs, (S)VCDs and XCDs on the fly, and it adds support for 3GP, AAC, AC3, APE, AVI, DivX, 3ivx, DAT, h.264, x264, AVC, Nero Digital, DTS, FLV, FLAC, HD-MOV, MPEG-1, MPEG-2, M4A, MPC, MP3, MP4, MO3, MOD, MKV/MKA, MTM, OFR, TTA, OGG/OGM, S3M, Vorbis, VOB, WavPack, ATRAC3, XviD, XM, WV, and UMX.