http://akismet.com/
Akismet is a free for personal use comment spam web service. It's highly effective as the database is updated by the users to provide protection against the latest spam sweeps. You never have to update your own database to have the best filtering.
It's similar to Spam Assassin where you can submit spam to be added to the filter or ham false positives.
It's available in plug-in form for a number of web applications or available as an API for a wide number of programming languages (like PHP and Python). Implimentation is easy for both routes.
I've been using Akismet for a week now. It's blocked 20 comment spam messages from making their way onto my site. It's done this without any false-positives or interfering with the visitor. Forget the unaccessible CAPTCHA nonsense that users hate. Use Akismet. I'm very happy with my choice and recommend it to anyone that has a comment system on their site.