iriverter - convert your videos for multimedia players

iriverter is a frontend to mencoder (comes along mplayer) that lets you convert videos for various multimedia formats like H320, H340, PMP120, PMP140, U10, iAudio X5, iPod Video

iriverter is available for Windows, OS X and Linux

On Ubuntu - use - sudo apt-get install iriverter

For others download from the iriverter site