Perhaps some Trust Metrics can be used to implement a Karma system
This metrics, used in Google-rank or in Advogato, try to issue exploits and attacks.
Thank you, Elnopintan, for stealing the thoughts right out of my mind! But I'll amplify a bit for the sake of the readership.
As I see it, what I've been wanting in a game system is not a tool that tells me what the community thinks of someone, but what I'm likely to think of someone. The community as a whole very well may be moving in a direction I don't enjoy: over-competitive (or under), obsessed with certain behaviours or avoiding certain tactics, whatever. I don't care about them. What I do care about is rating folks after a game based on my involvement / fun / approval and then having the system take my opinion as paramount and the opinion of my Friends / folks I've ranked highly, and then extrapolatively rank folks as I might based on my historical tastes and what people I consider meaningful to me (ie. those I had fun with) rated them.
In a system like this, "gaming the system" is very, very difficult. A Web of Trust matrix exists as a set of connections, so if me and my friends enjoy playing together, we'll be mutually rating each other highly. Doesm't matter to the rest of the matrix, we can be perfectly happy doing so. If one of my friends ventures out of the group bubble of ratings and ranks someone he found in a pick-up game well, then the web links up, and a friend of a friend'll be rated somewhat more favourably (or poorly if its a negative rating!). This lets people get more of what they want and less of what they don't, and that's a total plus.
Automated ratings based on karma from "completed / won / lost" games are a wholly orthagonal issue. I'd rather play with someone who may have disconnected 5 of his last 10 games (due to network ick, maybe, or whatnot) that my friends consider a good rating than a guy that rides every game out to the bitter end but is a complete douche to game with or who exploits every tiny exploitable ever. That is the kind of information I need.
Mechanically, all it'd take is a post-game thumbs-up, thumbs-down or "do nothing" button next to everyone's name. You don't have to rate anyone, but you can. And that data goes into the database, and when a new person appears in front of you, walk the web and see what trust level they possess. (It's usually implimented as some kind of spreading activation network but any kind of digraph would do just fine ...)
There's a web-link sharing implimentation of something very like over at Jaanix. I'm sure if there were any questions about underlying collaborative filtering algorithms, Joe'd be happy to help.