How do I get my DVD movies into my AppleTV, iPhone or iPod etc? This is a question I receive frequently. Altho there are many tools to accomplish this, I prefer using Handbrake. HandBrake is an open-source, GPL-licensed, multiplatform, multithreaded video transcoder that works in Windows, Mac, or Linux.
It converts your DVD's into iPod-friendly MPEG-4 or H.264 video files. The application includes presets for iPod, iPhone, Apple TV, PSP, PS3, and Xbox360. You can also choose various audio and video settings for a custom output.
If you want to convert other existing files such as AVI's, MKV or MPG's, Handbrake handles those as well now. However for straight file conversion between video formats, I use Video Monkey (Previously VisualHub). It's Mac only, but there's almost no format it can't handle.