Asign each site a tracking number. Then when you give them the code add it to the end of your url like so, some.domain.com/index.html?123. That way each individual site has its own built in tracking code. Your referrer logs will keep track of the individual ids for you.
You can also give each visitor a number, then add (when they visit the site) the referer in a database (include the resolution, hostname, ip address, browser version etc.) and you can search in your database to a number, then you see the other things. You can also sort things (from which sites the most visitors came) and so on.