This morning I've been playing with the coolest toys: Bump, and Bump Pay apps (those are iPhone app links, but they're also available for Android). With one fist bump you can swap photos, contact info, and money. Assuming everyone you know has a smart phone, you'll never have to drag cash around again. The video is almost unbearably schmaltzy, but it shows you, more or less, how it all works. I love technology.
I've seen tech review complaints about having to be in fist-bump range when making payment, but that's one of the things I like; it feels like a handshake. It's personal.