Miro Video Converter can convert almost any video file (AVI, MOV, WMV, Theora, FLV etc) to MP4, Theora, or MP3 (the audio only).
It has presets that will convert video to the correct sizes and formats for popular phones, iPhone, iPods, and other media players.
The best part is, it has very simple interface.
Simply drag & drop or choose your video and then the device/format you want. Then click Convert. That's all it takes.
Check it out, here's the link:
Miro Video Converter