Mateu Batle at Collabora has been doing this interesting real-time video editor that takes advantage of the video support in the HTML5 specification. It's working in a WebKit-based browser.
The current WebKit extension is implemented using the GStreamer Editing Services (GES) module. Making this video editor possible is also jQuery.
For those that want to learn more about this Collabora HTML5 video editor that's currently in development or to play with the code, visit this blog post.