Dwolla lets you send money to your Facebook or Twitter friends for a small flat fee
A very cool thing about this service is its flat fee. It charges the receiver by default (although the sender can opt to cover the fee), and the fee is always a low $0.25, regardless of the amount being transferred. Also, that's the only fee. There are no fees for automatic withdrawals, manual withdrawals, or anything else.
One downside is that Dwolla is only available in the US for now. Hopefully, they will at some point decide that the world is worth conquering, but until then, only those in the States can use it.