Although they do charge a small fee, something like 2.9% + $ .30 (don’t quote me, I’m just the messenger). With this new option, link your credit card and bank account with PayPal and pay your friends without needing cash.
Mashing-up with Bump, allows users to make paying people just a bit more entertaining. My buddy needed to pay me for a parking validation the other day so we "bumped" phones. Bumps app recently added Facebook integration, the list goes on.
App developers are starting to get very creative with collaboration mash-ups that shared the best technology with the best features and design. Another great example is Occipital, makers of Red Laser. From their site "Occipital, LLC is a technology startup focused on mobile visual search, founded by University of Michigan alumni Jeffrey Powers and Vikas Reddy."
I think it’s great that although the iPhone OS is essentially a close system, developers are teaming up to share and license technology.