I would recommend VLC for OSX previewing. VLC: http://www.videolan.org/vlc/ (choose nearest mirror site) But i use QT for all mpg formats since I already have mpeg 1-4 plug-ins for QT, & I use VLC for everything else. Don't forget to update to 3rd party plug-ins on the update page.

Also, DivX, - XviD, - 3ivx , etc.