Two picks on one day?! Well, I was just too excited about this one to not share it immediately!
Perian isn’t anything terribly sexy or cool. It’s a simple Quicktime component which adds support for a variety of codecs to the Quicktime player. (Specifically, XviD, DivX, AVI MPEG-4, MS MPEG-4, 3viX and more.)
Why not just use VLC?
Well, like VLC, Perian offers easy smooth playback, and simple installation, but it also builds all this video support into Quicktime, so that it works with Front Row and any other video player based on Quicktime.