XviD
XviD is an open source (under the GPL) implementation of an MPEG-4 Visual encoder and decoder.
It was developed to be a free & open-source alternative to the commercial & closed source DivX.
It supports MPEG-4 Visual Simple Profile (which is the only profile QuickTime supports) and Advanced Simple Profile, that includes improvements over Simple Profile like B-frames, General Motion Compensation and Quarter Pixel.