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XviD is one of the most developed MPEG-4 compatible codecs. It's frequently thought to be the best MPEG-4 compatible codec out there due to its development (GNU open-style). It's primarily targeted at Linux/*nix platforms, although at least 3 people keep updated Windows builds available.
It supports (in development builds) many of the more advanced MPEG-4 features, such as bidirectionally predicted frames (so-called B-VOPs/B-frames), quarter pixel motion search (QPel), chroma motion search (helps a LOT for chrominance-heavy and luminance-light scenes), extremely precise motion searches, lots of knobs (for the knob-turners, you know who you are ;), and the beginnings of General Motion Compensation (GMC, used to detect pans, zooms, etc. and encode accordingly) have already appeared in it.
Results such as getting a 21.6 megabyte file for a movie 1:26(23.976fps) long at 768x576 with close to no MPEG artifacts are not uncommon with XviD, although how well something compresses is purely dependant on the source material.
XviD codec can be downloaded from:
