Rewind Theater lets you draw shapes on videos to highlight things on screen as the video plays or slowly step forward or backward in a video. It was inspired by IGN's Rewind Theater videos, the difference being this app lets you do it live (but does not save your annotations).
It uses HTML5 video, so its support for different video formats depends on your browser. It supports mouse or touch for drawing, as well as keyboard shortcut alternatives for the on-screen buttons.
The initial version of the app was written in under 48 hours for the event where it was first used. Thus, while it functions fine, it may not be number one in user-friendliness.