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.
