Joe Berkovitz created ReviewTube, a Flex application that lets you write captions for YouTube movies. Joe explains the app online (and makes the source code available), and will be discussing design patterns and best practices involved in creating the app in his MAX session.
You an use http://tubecaption.com to add your subtitles, captions, comments etc to any YouTube video.
Try the demo here http://tubecaption.com/captions/demo
You can also earn money with the captions you add to YouTube videos.