I am very impressed with the Akismet Spam filtering system that WordPress has integrated into their blogging software. Recently this blog has gotten quite a bit of spam and Akismet has stopped nearly all of it (99.996%). I moderate the comments before they are posted here and only 2 of 450+ spam comments were in my moderation list. I'm not sure why they weren't in the spam folder as they were clearly spam, but I am still thoroughly impressed.
I haven't upgraded to WP 2.1 yet, but I hear that there is an updated version of Akismet included in the update. Akismet is free for personal use, and if you have a WordPress blog, I urgently recommend using it.
