You can just use iTunes, which is already installed on every Mac, can play anything QuickTime can play (including WMVs via Flip4Mac), and also keeps your files organized all nice and pretty. The iTunes full-screen also has really a really nice controller that appears when you move your mouse around.
I'm stunned every time one of these threads comes up and nobody suggests this.
Or you could use VLC, which isn't as pretty, but it'll play just about anything (except wmvs) in any viewing mode you like. And it's free!
i hate how iTunes handles videoplayback. First it starts in that little corner screen, which you click, which opens a separate window, etc...
iTunes is my least favorite way to view videos... each to his/her own!
Since when has iTunes been able to play .avi's encoded with divx? I've tried importing some .avi's and it wont have it, even with the correct quicktime components installed.