A few weeks back I came across this great site from Larry Ferlazzo's blog. Now that I had the chance to explore it and watch some tutorials I've decided to post about it. Remix America is simply one of the best online resources for history teachers/students.
RA, is a tool which lets you take any video from America History (from their own, online resources, or your own) and then mash them together anyway you see fit. You can combine the videos, upload audio, and add transitions all from their web interface. Once it is done, you can either upload your video to their site, download it to your computer, or get the html code to embed. However, while RA is a very good at what it does, it is limited to only American History. Also, keep in mind since it is a Web 2.0 resource that you have to be careful when sticking to CIPA compliance. I've contacted RA to get more information on that in detail.
Below is a "tutorial on how-to make a remix".