Everyone gets to be a commentator with this assignment I give my television production students. The task is to find three video clips- shoot them yourself and "rip" something from YouTube (keepvid.com is a great site for doing this). Select things you are interested in talking about.
Then use Apple iMovie's picture-in-picture tool, insert your commentary over the clips. The Mac computers make the process easier by having a built in camera. Activate that, speak your mind, and the on camera segment goes directly into your iMovie editing project.