For great videos on the go with you iPhone, iPod Touch, or Android... use Handbrake! Yes, it is an odd name for an app... with an even odder icon... but it works great!
One step video conversions to allow easy playback on your iOS or Android device.
Some would prefer downloading an app that plays all formats, but I don't really like wasting storage space with a 15GB movie, when you can't really appreciate 1080p on a mobile screen.
The iPhone 4 preset works on almost all devices, or create a higher 720p preset if your mobile screen supports it.