Perian

Perian is a free, open-source QuickTime component for the Mac. It allows you to play video formats that are not natively supported by the Mac on QuickTime and any other player that uses QuickTime, such as iTunes or Front Row.

This is not a video player, and it doesn't have a graphical user interface other than a few settings in a preferences window.

The video formats that Perian allows you to play in Quicktime are MPEG4, DivX, 3ivx, H.264, Flash, VP6, H263, MPEG-1, 2, and 4, among others. Perian can also change the way QuickTime processes audio. There is a drop-down menu under the preferences window that allows you to set Dolby Pro Logic, Dolby Pro Logic II, Stereo or Multichannel as audio output.

Another great feature that Perian makes possible is external subtitles. When you have Perian installed and place an SRT file with the same name of the video file that you are trying to play, Perian will load subtitles and show them. There aren't any size or font options for them, but I found that they look good and work well.

Perian works well and doesn't slow the video playback process at all. Watching AVI files under QuickTime with Perian is a very nice experience and if you really like QuickTime, it is the way to go.