Three Cheers for Perian
Run , don’t walk, to http://perian.org/ and install Perian, a simple, Mac-friendly collection of open source video codecs that finally lets QuickTime play and encode many popular video formats that Apple has ignored including Divx, XviD, FLV, AVI, MS-MPEG4 v1, MS-MPEG4 v2, MS-MPEG4 v3, DivX 3.11 alpha, 3ivX, Sorenson H.263, Flash Screen Video, MKV, Truemotion VP6, and AVI.
Yes, I know there are other products out there like VLC that can play some of this stuff some of the time. This is the first one that actually works.
Definitely worth a donation.