Recently, I got a task of building a P2P file sharing system.

There is one requirement : the system should have a document ranking algorithm so that it could be used help users find more valuable files.

Several strategy might be useful :

• force user to give a score to a file before it was downloaded

• The documents containing certain key words would get higher rank

• Manager could modify file ranking manually

• the more a file was downloaded, it would get a higher rank.

Do you know any other strategy or methods that also appropriate? Or is there any real world example?