Perian 0.5

Perian is a free plugin for QuickTime that enables it to play almost any type of 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."

Perian v0.5 [download] (948 KB)

PS: If it doesn't work, unzip the file and copy it to "~/Library/QuickTime". (You may have to create that folder)