There are lots of editing programs, linear and nonlinear, from really simple all the way up to the stuff Dreamworks and ILM use. I know Dreamworks and ILM have both contributed to Cinepaint, for example, but since I don't do anything but EDLs I don't know what the pro level tools are.
Check out Ubuntu Studio, though:
http://ubuntustudio.org/
For rendering and conversions there are a ton of GUI front ends that run ffmpeg and mencoder behind the scenes.
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