Cinelerra is a free and open source software non-linear video editing system for the Linux operating system. (However, it has also been successfully ported to Mac OS X.) It is produced by Heroine Virtual, and is distributed under the GNU General Public License. Cinelerra also includes a video compositing engine, allowing the user to perform common compositing operations such as keying and mattes.
Cinelerra, one of the only serious video editing and compositing tools available for Linux, can cause frustration for users trying to install it on Ubuntu Studio. Fortunately, after several attempts, I found a way to install it easily.