Perian is a free QuickTime component that ads support for numerous file types as well as numerous video and audio codecs.
Supported file types includes: AVI, DIVX, FLV, MKV, GVI, VP6, and VFW
Video codecs: Many variants of mpeg4 (DIVX, 3ivx, MS-MPEG4 etc.), H263, H264, Techsmith Screen Capture and many more.
Audio codecs: Windows Audio I & II, Xiph Vorbis, Mpeg I && II audio, AAC, AC3 and more!
If you are using an Apple Macbook, Mac Mini or an iMac then this is the solution for playing back AC3 encoded audio, and for playing back AVI files in OS X.
Perian – AC3/AVI support in Mac OS X was added to MovieCodec.com for AV Recievers (10-21-2009).