FFDShow MPEG-4 Video Decoder - A Freeware DirectShow decoding filter

FFDShow MPEG-4 Video Decoder is a DirectShow decoding filter for decompressing DIVX movies, picture postprocessing, and show subtitles. It uses libavcodec from ffmpeg project or for video decompression (it can use xvid.dll installed with xvid codec too), postprocessing code from mplayer to enhance visual quality of low bitrate movies, and is based on original DirectShow filter from XviD, which is GPL'ed educational implementation of MPEG4 encoder.

The best thing about FFDShow is that it is a Freeware and an Open Source Program.