list of open decoder:
1-jm: its standard decoder and complet(it has encoder too)
http://iphome.hhi.de/suehring/tml/
2- ffmpeg:FFmpeg is a free software project that produces libraries and programs for handling multimedia data. The most notable parts of FFmpeg are libavcodec, an audio/video codec library used by several other projects, libavformat, an audio/video container mux and demux library, and the ffmpeg command line program for transcoding multimedia files.
http://ffmpeg.org/
3-xvid:Open-source compression codec based on MPEG-4 ISO format.
www.xvid.org
4-ludh: udh264 is a H264 video decoder. It has been written from scratch in C, with a constant idea in mind: multi-threading.
http://www.ludrao.net/home/ludh264
5-basic basic H.264/AVC decoder by Martin Fiedler
http://keyj.emphy.de/projects/studies/