Kmenc15 is a great KDE front-end for MEncoder. It allows you to convert MPlayer-supported video into formats such nuv, divx, xvid and qtvideo. It also has a very cool feature which is nice if you do basic video editing: You can select parts of a video file and convert that part into a new video file.
KMec15 also has options for cropping video, applying various filters like noise, flip, mirror, etc.
It really is a nice must-have GUI front-end for video converting on Linux.