DivX for Mac is a video compressor for the Mac OS, includes player and converter software
From the official site:
DivX® 7 for Mac is a free download that provides everything you need to enjoy high-quality digital videos on your computer, including high definition H.264 (.mkv) videos with AAC audio and videos created using all previous versions of DivX technology. You can also play your DivX files (.divx, .avi) on millions of DivX Certified® devices today. Look for DivX Plus™ HD Certified devices soon that will support playback of all DivX and DivX Plus HD files, as well as .mkv (H.264/AAC) videos from the Internet.
There are also other version of the DivX codec, available for different operating systems :