A few months ago I installed the Challenge plugin, which presents a math question for anyone wanting to post a comment. It's been working great and I have not received a single spam comment... until this morning. I woke up and checked my Blackberry for new email: 49 new email messages. They were all comment notifications. Apparently a spam bot managed to get around the Challenge plugin and posted 49 spam comments throughout my blog!
After deleting the spam, I decided to install the same plugin I've installed on a couple of other WordPress blogs, SecureImage (the developers site is down and that is the best URL I can find for SecureImage). It's a simple captcha plugin that generates an image containing a string of letters. Other captcha plugins required GDLib, which I couldn't seem to find on my web server (or at least didn't bother spending much time looking for it). SecureImage requires ImageMagick, which is readily available (read: default
/usr/bin/convert path worked fine). After installing a TrueTypeFont (FreeSansBold.ttf) I was up and running.