Perian is a free and open source QuickTime component that adds native support for many popular video formats.

Here are some key features of "Perian":

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.


Requirements:

· To play WMV files in QuickTime, get Flip4Mac.