This is probably one of the most interesting projects I have worked on thus far. To work on it I needed to rapidly learn a new platform (Node.js) and other new-to-me technologies.
This website, originally set up as a contest, allowed users to select a series of video clips for the purposes of making their own tourism promotional video. Once the user is finished selecting their video clips, an AJAX call is made to the server with the id numbers of the selected videos. The server then uses FFmpeg to stitch the videos together and overlay a background audio track.
This was my favourite project at the time because it really challenged my perceptions of what I was capable of building on the internet and required me to learn several new technologies while building it.