Referrer Karma is the latest tool from Dr. Dave, the creator of the ultimate spam plugin, Spam Karma. As myself and many others have experienced recently, referrer spam can virtually take down a server. I experienced it live the other night as well as Mark from Weblog Tools Collection.
Referrer Karma is not a WordPress plugin but will work with WordPress or any other PHP page. Similar to Spam Karma, Referrer Karma decides whether to allow a referred inbound link or not based on specific rules. This is an alpha version and not for the PHP newbie.