Reputation and point system models
O'Reilly and Associates actually has a blog and a book coming out about this.
As far as 'systems', there are many:
- Slashdot
- Digg
- Wikipedia (Implicit with Trusted editors)
- Ebay
Among many, many others.
You can check other sites like this one (expert-exchange) - but I think SO is the best one I've found on the net.
For a related but different approach, check out http://www.klout.com - they're building a web-wide reputation system that looks at social signals from a growing list of sites.