PhotoSwap is about a month old, and was written up recently in Wired. The idea is unique — you take a picture with your iPhone, submit it, and you receive a random picture back. Generally, the pictures aren't particularly interesting (computer screens, offices, apartments, etc..), but some are quite innovative. What makes it particularly interesting is that you can "reply" to a picture by taking another picture. You can also (sometimes) see where the senders are on a map.
There are a couple groups already on Flickr containing SFW and NSFW galleries. The reviews on App Store are generally very positive — some people claim that this is the single best app around. It has the feel of a very very early Myspace — where you don't quite get it, but you can't stop looking.
The app needs several features such as the ability to save the pictures you receive, bookmark people, and segregate conversations and photos in rooms. Stay tuned: We will be following the developer Padadaz over time.