Format Junkie is a handy GUI tool to convert files from one format to another. It handles audio files, video files, images, iso files and other stuff. The project is hosted here: https://launchpad.net/format-junkie .
*** Bug 434154 has been marked as a duplicate of this bug. ***
depends on Ubuntu's libunity. :(
Oh, I'm running it on Linux Mint "Mate Edition". I believe the libunity is just necessary because there are some 'advanced' options to enable progress bar and something on unity panel - aka unity integration.