The Video Acceleration API (VA-API) was originally developed by Intel and can be found partially supported in some Intel Linux graphics drivers (such as with the Clarkdale / Arrandale IGPs) and also happens to be the API by which AMD exposes their UVD2-based XvBA implementation. Splitted Desktop Systems has also written a VA-API front-end that then uses VDPAU. S3 Graphics also supports VA-API.
As of commit 29053 the VA-API support for XBMC should be working and will then officially be found in the next official release for this popular software project. Other projects already supporting VA-API include MPlayer and FFmpeg, Fluendo's new GStreamer codecs, and even the FSF Gnash Flash Player.
This is good news for those with a graphics processor and driver that support using VA-API and are looking to ease up the video playback process from your CPU and move more of that work over to the GPU, which is particularly useful with HD video files.