The iPhone is a wonderful device. However when it comes to play videos it turns out to be very picky.
Indeed, the iPhone only accepts 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 or iPad.
It is extremely easy to use, free for non-commercial use and does NOT require your iPhone/iPod/iPad to be "JailBroken". More Info »
Movies2iPhone currently runs under Windows Xp, Vista, Seven (32 & 64 bits) and Mac OSX Leopard (10.5) and Snow Leopard (10.6).