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.

Note: Perian requires 10.4.7

http://perian.org/

The inclusion of this software link is not a recommendation.
I have no personal experience of this software.
It appears to bring useful features.