A frontend to launch VLC, follow someone on Twitter, and display any live @tweets mentioning that account as subtitles in VLC.
Geographically separated people can schedule a video to play simultaneously, follow the same account, compose tweets, and see each other's commentary. (Example: MockTM Events)
Alternatively, if the video isn't playable in VLC, commentary can be sent to a window that can float over proprietary players.
Windows, Linux, or... possibly OSX (coded w/o a test box).