A video codec designed by On2 Consulting.
Available for Windows (Video for Windows) and MacOS (QuickTime).
The current license is under a basically DFSG-incompatible license (MPL with restrictions). However, On2 and Xiphophorus are working on Theora, a version of the codec that has less license restrictions, more features, and better support for the Ogg bitstream container format. Once the Theora bitstream is feature frozen and tools are developed, VP3 bitstreams can be losslessly converted to Theora bitstreams.
According to the hype, VP3 is as good and usually even better than the proprietary Sorenson codec that people often blame for QuickTime's failure in open source world. The codec was licensed by both RealNetworks and Apple back in the day, which probably tells something about its quality.
From cursory tests, when using quick compression mode it appears to be at least as good as Indeo 5 thingy at about the same bitrates... to me, it didn't seem like utter crap =) However, it didn't exactly seem to me that it'd beat XviD at the same bitrates, but I think I should blame my incapably set settings. Maybe it is worth something.
(Information here taken from the home page; I will tell more when I have had a chance to get an in-depth look at it...)
mplayer's and XINE's current versions seem to play VP3 .AVIs just fine (with help from Windows DLL, though).
Home page: http://www.vp3.com/