Try Cinelerra.

Cinelerra is the most advanced non-linear video editor and compositor for Linux. Cinelerra also includes a video compositing engine, allowing the user to perform common compositing operations such as keying and mattes.^Cinelerra

To download click here. (Edit: or install on 12.04 with the following commands.)

sudo apt-add-repository ppa:cinelerra-ppa/ppa

sudo apt-get update

sudo apt-get install cinelerra-cv

Also you can try OpenShot.

OpenShot has many great features, such as trimming and arranging videos, adjusting audio levels, transitions between videos, compositing multiple layers of video, chroma-key / green screen effect, and support of most formats and codecs.

What really sets OpenShot apart from other video editors is the easy-to-use user interface.^OpenShot

To install OpenShot, just press Ctrl+Alt+T on your keyboard to open Terminal. When it opens, run the command(s) below:

sudo add-apt-repository ppa:openshot.developers/ppa

sudo apt-get update

sudo apt-get install openshot openshot-doc