Dwolla.com is a new payment network. It makes it easy to transfer money without the transaction fees charged by credit cards. Each transaction over $10 is 25 cents, and those under $10 are free. I think this has a lot of potential so I have been exploring ways to utilize their API.
They have some interesting API methods that are not common to payment APIs. You can view a user's account transactions and balances (with permission of course). They also integrate and expose a user's contacts from various social networks, such as Facebook and Twitter, to make it easy to send/request money from friends.
Let me know if you build something cool or if you have a great idea that could utilize their API.