Akismet is a fantastically effective Spam blocker primarily used by WordPress Blogs to stop Comment spam. The latest Akismet plugin (Or maybe I never noticed before!) has fairly comprehensive stats on what its caught, missed and generally how effective it is.
My Akismet has caught (since I turned it on) 7,980 spam comments with an accuracy of 99.616% and only a handful of false positives so I’m happy! Isn’t it amazing though that 94.39% of all my comments are spam!? Bastards.
Make sure you turn yours on!