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. You can obviously record yourself using it, but the app does not modify the videos or save the annotations (or any other user data) after closed.
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, back in 2013, so while it functioned fine, it was definitely not number one in user-friendliness. In 2021, I revamped the experience with better icons, video loading times, and help tips, as well as offline functionality and some small flourishes like the ink ripple effect when switching tools.