The iPhones, iPad, iPad 2 and iPods are a wonderful devices. However when it comes to play videos they turn out to be very picky.
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 iPad 2.
It is extremely easy to use, free for non-commercial use and does NOT require your iPhone/iPod/iPad to be "JailBroken".