http://www.3ivx.com/codec/3ivx_d4_403_mac.sit

3ivx video is a modern MPEG-4 video compression system which 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 3ivx decoder produces noticeably higher quality video than other decoders due to its high quality post-processing filters, and because of its core design and optimization maintains a record-breaking decode speed. 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 industry-leading compact 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.

What's New:
This version adds the following:

More Speed
More AltiVec
More Filters
More Features
More Quality
Global Motion Compensation (GMC)
B-Frames
Quarter Pixel Motion Compensation (QPel)
High Speed MPEG-4 compliant QuickTime Encoder

does it runs at ffmpegx too?

MACpenta