My wife, brother and I have been playing with Photoswap by Padadaz for a few days now and it is now one of the most used applications on my iPhone.
You take a picture and send it out. It is received by one and only one person. If they find it interesting they can directly reply to you with a picture of their own. Thats it.
I’ve personally had a ‘conversation’ last for over two hours, sending pictures of random toys in my office and getting random toys in return.