Whuffie
In the Cory Doctorow novel Down and Out In The Magic Kingdom, the author introduces a concept he calls "whuffie", which Wikipedia describes as an "ephemeral, reputation-based currency".
How it works is simple. Do good things to other people, or help someone out, and your whuffie goes up. Let someone down (or worse) and your whuffie goes down. The higher your whuffie, the more likely strangers are to want to interact with you in a positive fashion, based on the assumption that the community considers you to be a good individual. It's a step up from "you scratch my back, I'll scratch yours" to "you scratch my back, someone else will scratch yours".
The areas of application for such a system are limited only by our imagination.
Needless to say, it was just a matter of time before someone got round to implementing the idea in real life. Visit The Bitchun Society to find out more.