A client of mine, has a very popular blog which he requires users to register to post comments. Lately, he has been inundated with spam users and was asking for a fix.
If you are requiring users to register to post comments, and not using a plugin like Disqus or IntenseDebate then take a look at this plugin.
http://wordpress.org/extend/plugins/stop-spammer-registrations-plugin/
It uses a couple different places like stopforumspam.com and Project Honeypot to weed out the bad guys. So far it seems to be working as the registered user requests from spam bots is eliminated. The human spammers ones are down too since it can recognize their IP address from the databases.
As always, make sure you back up the site before deploying.