CoreAVC is a proprietary Windows codec for H.264 video decoding. It is much faster than any currently available open-source codecs. Being multi-threaded, and able to play PAFF streams, it can handle HD H.264/AVC streams that no freely available codecs can. CoreAVC is reasonably priced at ~$15, but it is Windows only. Various patches are available here to enable loading CoreAVC into 32bit and 64bit linux players.
NOTICE: I am direly in need of help. I no longer have the time to work on this project much, and there is quite a bit to do. If you think you can meaningfully contribute, please contact me.
Yes, we're back. CoreCodec has given their blessing to this project.
Also, the patches have been updated for 1.7.0 support (except xine), if someone wants to make a xine patch, please just send it to me and I'll update it.