Subyo lets you create subtitles for any YouTube video, and search for subtitled videos. It seems similar to dotSub, which Scott McLeod recently reminded about, though dotSub, I believe, doesn’t limit itself to YouTube videos. 

Of course, the fact that most School Districts block YouTube diminishes the potential educational use of these sites.  I haven’t really explored either one of these applications, and I’d be interested in hearing from people who have.