YouDescribe is a free, web-based tool that allows anyone, anywhere to add audio descriptions to any YouTube video, and share them quickly and easily. YouDescribe allows describers to record brief descriptions that are stored in the cloud and played back when the viewer watches the YouTube video with the YouDescribe player.
Here is a quick promotional video made for YouDescribe:
http://www.youtube.com/watch?v=XwKm4RILZa4