Editor One is a system used by many educational institutions, who purchase it. However, they make a “demo” version available to anybody for free, and it’s pretty easy to use. It has a fair number of clips you can “mashup” and use to create your own video.
I’m adding it to The Best Ways For Students To Create Online Videos (Using Someone Else’s Content), which I recently updated.
Thanks to the Make Use Of blog for the tip.