Konttori's Media Converter (https://garage.maemo.org/projects/mediaconverter/), although it doesn't work correctly for me every time (I have off and on problems with aspect ratios, but other people love it unconditionally).

VirtualDub (http://www.virtualdub.org/index) is something I use regularly. It's more fiddly than Media Converter and the converted movies only play in Mplayer (Media Player is very finnicky about the number of pixels of a videoframe and what kind of Divx flavour is used), but Mplayer is at the moment way better than Media Player anyway.

There is a Perl script somewhere, but (with humblest apologies to the author) I can't juggle my brain into the right gear to find it.

Check the howto's on the maemo.org site.