Perian is a free plugin that enables QuickTime to play almost every popular video format.
With Perian installed, any OS X application that uses QuickTime can now use these additional media types:
Divx, XviD, FLV, AVI
MS-MPEG4 v1, MS-MPEG4 v2, MS-MPEG4 v3, DivX 3.11 alpha, 3ivX, Sorenson H.263, Flash Screen Video, Truemotion VP6
These formats when they are inside an AVI:
h.264, mpeg4, AAC, AC3 Audio, and VBR MP3.