New networking technologies such as wireless mobile grids and peer-to-peer middleware are examples of a growing class of open distributed systems whose strength is the absence of a central controlling instance and which function through the cooperation of autonomous entities that voluntarily commit resources to a common pool. The social dilemma in such systems is that it is advantageous for rational users to access the common pool resources without making any commitment of their own. This is commonly known as "free-riding." However, if a substantial number of users followed this selfish...