Perian - play almost every popular video format with quicktime
Perian is a free plugin that enables Quicktime to play almost every popular media format. Except WMV, any Mac OS X applications that use Quicktime will support popular formats such as DivX, XviD, FLV, AVI, and many more.
Install perian along with Flip4Mac and you can now use Quicktime instead of VLC Media player.