I’ve been getting a lot of comment spam lately, so I’ve installed Bad Behavior, which takes a different approach to spam blocking:
Bad Behavior was designed and built by watching actual spambots which harvested email addresses, posted comment spam, and used fake referrers. By logging their entire HTTP requests and comparing them to HTTP requests of legitimate users, it is possible to detect most spambots.
Let me know if you have any problems. I’ll post some impressions after I’ve used it a bit.