AVI is a wrapper for all types of movie files. It's not so much an AVI codec, but the video encoding codec that counts such as DivX, XviD, 3ivx, OggVorbis etc.
The DivX codec for Quicktime for Tiger and QT7 is still currently in beta as the Fusion Beta 3 codec.
VLC (VideoLAN Client) is very good though as well, that said, I kind of prefer to use Quicktime, as I have QT Pro 7 and it just has a slicker UI.
Vicky