Perian is a really handy tool if you have to deal with flash videos(.flv). With this Quicktime plugin you can simply watch those movies inyour Quicktime player.

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 – A swiss-army knife for QuickTime

And the best: it’s free (as free beer) and open source (licensed under GPL 2)