If you have Quicktime Pro 7.0.3, you can take any complete non-streaming video compatible with it and convert it to a video capable of being played on the iPod.
1. Export the file with the File menu of the Quicktime player and select Export's popup menu.
2. From the popup menu select Movie to iPod (320 x 240). Give it a name that you know means it is ready for the iPod.
3. Save the resulting m4v where you can find it in the Finder.
4. Drag it into iTunes 6.0 or later. It will then appear in the Video subsection of the source.
5. Then in the iTunes menu -> Preferences -> iPod -> Videos select automatically update all videos. Or select a playlist if you added videos to a playlist. And then click OK.
6. If the iPod does not update right away, control-mouse click (or right click if you have a multibutton mouse) and click on Update. Your video should appear in the Videos menu of your iPod on your next update.
Note: the version numbers of software are the ones I tried it on. I don't know if it is compatible with older versions or will work with older versions.