Windows: Many Lifehacker readers use Handbrake for converting media to watching on tablets and smartphones, but even with a graphical front end it can be a bit overwhelming. If you want a simpler means of converting video files, RockPod 2 is a straightforward way of doing it.
The converter was developed by the team at Australian PC User, which means that only the older version is offered for free online -- the newer version 5, which has a wider range of options such as audio stripping and converting ISO files, is available on the cover disc of the current issue. That said, the older version still covers most of the options you're likely to want, including presets for a wide range of popular devices.
RockPod 2 is a free download for Windows only.