What makes Mover (click opens iTunes) so cool is the way that it does the transfer. When you want to transfer an image or a contact card to another iPhone, you just launch the app on both devices, grab the image or card, point an arrow on one screen to match up with an arrow on another iPhone screen, then flick an icon representing the data you're sending. It moves off of your screen, and onto the other iPhone's screen, and the data is saved automatically. Very cool, and very iPhone-like!
Both iPhones need to be on the same WiFi network; there's no current way to do this over Bluetooth or a cellular network connection. Emanuele has made his code open source under the BSD license, so other developers are welcome to take a look at how he did this.
Check out Mover here (now with the correct embedding info):