LETSystems
See http://www.gmlets.u-net.com/design/home.html for an overview and http://www.gmlets.u-net.com/ for more info.
Timelets:
In the overview they use national currency as measurement for value.
But I think it would be mush better to use time for this.
If I need a friend to help me for 2.5 hours, I could just pay him 2.5 hours in timelets.
This would also prevent inflation and make it global acceptable.