This project contains two separate interfaces for Windows and Mac OS to download. They essentially do the same thing.
Select a source folder with some video clips (or several files in the Mac version). The program will change the container to the selected output without reencoding. For Windows, ffmpeg needs to be installed on your system, it is included in the Mac download.
For a short video demonstration of the Mac version, go to: http://www.youtube.com/watch?v=O278FHawa9k