Akismet is a great SPAM buster
I’ve been using the Akismet wordpress plugin to block comment spam and it has been fantastic. It is free for personal use and out of over 4000 spam comments that have been posted to my blog, Akismet has only let about 20 slip through to the comment moderator panel. Even more importantly, I’ve never seen a non-spam comment get flagged. Without Akismet (or something similar) I probably wouldn’t have comments enabled since the administrative burden would be too much. I highly recommend Akismet to anyone looking for a comment spam blocker.
I looked into using Akismet for my company’s blog, but for very small commercial sites, the price seems a little excessive ($600/year for 5 or less blogs).