I’m a bit late with this one, but the new official DivX codec for the Mac came out a couple of weeks ago, and integrates nicely with QuickTime. After the trial period there’s a charge if you want to create DivX-encoded movies, but the package also includes the free codec which will let you watch them indefinitely.
If you don’t know why you’d need this, you probably don’t! If, on the other hand, you regularly download movies created on PCs and find that they don’t play, this will help with quite a lot of them. The other solution is to install the excellent VLC player, which seems to play pretty much everything, though it isn’t as pretty or as easy to use as QuickTime Player.