You moight want to look into Credence.
It is a distributed Scam-watcher/File-Rating mechanism which is programmed into a modified LimeWire:
http://www.cs.cornell.edu/People/egs/credence/
You can rate up or rate down files, and your program checks, which other people rated the files like you did, and trusts them based on that information, so you get the scam-votings updated automatically.
It is in heavy developement, at the moment.