Registration Randomizer creates unique URLs for the registration page and checks that the referrer of the registration action matches these URLs in an attempt to prevent automated spammers from registering new users. It also tarpits IP addresses that fail the token check to prevent repeat attempts.

This plugin rewrites all registration links that use the output/url view. If your theme doesn't use this view, you will need to modify your theme before any registrations will work. You can use the output/registration_url view provided by this plugin, or you can see how to create your own links by looking at the code for that view.