Yahoo! has produced a guide to reputation systems with discussions about which system is right for your particular type of application. Some of the types described are: the competitive spectrum (points mean prizes); named levels (like rank for enthusiasts) and numbered levels (does what it says on the tin). As they say:

A person participating in a social structure expects to develop a reputation and hopes for insight into the reputations of others, but each designed model of participation and reputation embodies its own set of biases and incentive structures. Balancing these forces determines in large measure the success or failure of a social system.