The iPhones, iPods, iPad, iPad 2 and the New iPad are wonderful devices. However when it comes to play videos they turn out to be very picky.
Indeed, these Apple products only accept to play movies encoded in a very specific format: the so called mp4 format.
Movies2iPhone converts any video (Divx, Xvid, avi, mpeg, wmv, asf, mov, vob, flv (flash), 3gp, mkv, etc...) to a format optimized for your iPhone, iPod, iPad or 'New iPad' (iPad 3 or iPad HD if you prefer).
It is extremely easy to use, free for non-commercial use and does NOT require your iPhone/iPod/iPad to be "JailBroken".