The MSU Lossless Video Codec was developed by the Graphics & Media Lab Video Group of Moscow State University and claims to be, at least at its release, the codec with the "highest compression ratio in the lossless video compression field."
Unfortunately, testing is no longer available as the last version of the codec, 0.6.0, was released in 2005 and is not stable with Vista or 7.
Similar to SheerVideo and CorePNG but less popular.