The primary function of Tor, anonymity, is extremely valuable to many people around the globe. Unfortunately, it also makes Tor an easily abused tool for cybercriminals - among them spammers. There are several sites that have given up trying to reject interactions selectively and instead have taken to banning all traffic originating from Tor exit points. This is bad for Tor users.
What are some tools that site operators can employ to keep their site available to Tor users without too much additional maintenance or effort ?
(here is an example of one good discussion about this topic and how site operators using a particular server application are coping)