Ask HN: Is there an easy way to transfer money to people?

Say I'm at a restaurant and (to make it easy) one guy pays the bill, and I want to give him my share of $11.53 for the meal. Is there an app for my phone where I can just click on his Facebook picture or enter his email and transfer that amount to him?

Or suppose I'm at a bake sale and I don't have any cash on me. I can just enter the name of the person running the sale and quickly transfer $2.50, with a note for my records saying that it was for a bake sale treat.

The reason I ask is: I haven't found such an app, and if there is one, it's not well-publicized. It seems like an interesting idea for a side-project. Thoughts?