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)