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.