Perian, the swiss army knife for QuickTime

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.