The Ripple Project - Open Souce Payment System/Concept

Ripple is a project to build a new kind of monetary system based on the trust present in our ordinary social and business relationships.

A peer-to-peer network protocol for making decentralized Ripple payments between users on different computers, a payment system where everyone is a banker.

Connect to your friends, family, and associates and your credit with them becomes a fully-functional currency

Software Implementations of the Ripple Concept

* Ripplepay - the original implementation
* Shire Hours - Ripplepay clone
* Rain Droplet - Open decentralized community credit service based on Ripple

Ripplexchange
Forum for exchange goods and services using Ripple!

Different (more sophisticated) software same concept:

Multiswap.net - Reinventing free trade
A free platform for circular barter exchange. See a flash demo!

Great further multimedia sources about the concept of Ripple.
Very extensive list of  ComparisonOfPaymentSystems

What is Money? - A quick intro to the ideas behind Ripple pay