From 3ivx Technologies:
3ivx video is a modern MPEG-4 video compression system that allows high-quality video in unprecedented file sizes. The 3ivx decoder plays back most MPEG-4 variants (including DivX 3, 4, and 5, Apple MPEG-4, Philips MPEG-4, and XviD). The Windows version of the decoder includes an AAC audio decoder for Windows Media Player as well as a media splitter (to play back AVI, MOV, and MP4 files inside Windows Media Player). The 3ivx encoder produces high-quality MPEG-4 video and is distributed as a plug-in for QuickTime as well as a plug-in for video for Windows. This allows you to encode MPEG-4 video in AVI, MOV, or MP4 files (with AAC audio) with the encoding application of your choice.
3ivx has been tested to work from Mac OS 8.6 up to Mac OS X 10.2.3.