There are plenty of tools out there for converting videos files to iPod and PSP compatible formats but my favourite is 3GP_Converter. It’s a free tool that lets you select your desired output format then simply drag and drop your source files to queue them up for conversion. It’ll accept any source that you have an installed codec for e.g. DivX, XviD etc.
Download and extract the Zip file then run the setup file. Note that the setup does not install the files, merely sets up the config so make sure you have extracted them to a suitable location.

Choose your language and the output format you wish to use.

Choose your specific output settings and the output directory. Now all you need to do is drag and drop your source files. When finished the software will open iTunes and add the completed video file to your library.
The config files can be manually edited to tweak the settings. You can find them in \default_setting in the 3GP_Converter folder. After you change any config files you’ll need to run the setup again.
I’ve made a tweak to my config file to handle widescreen source files. If you don’t make this change the output will appear stretched. You can download my updated config file here (change the extension to ini). These settings give me respectable quality when I hook my iPod up to the TV.
If you’ve stumbled across this posting because you have a Play-yan device then you can download a Play-yan config file for 3GP_Converter here (change the extension to ini). You’ll also need a file called playan.fup in the root of your SD card to play MPEG4 on a Play-yan.