6. Spambot Stopper
(http://www.vbulletin.org/forum/showthread.php?t=294633)
This is a very effective plugin at stopping spambots from registering on your forum. The plugin analyzes the time it took for a new user to fill out the registration form. And if it’s less than X seconds, the user will get banned or see an error message (plus a handful of other settings).