Notes: the Defensio Anti-Spam, Bad Behavior, and Growmap Anti Spambot Plugin (GASP) plugins are no longer available. I recommend Antispam Bee instead.
The Defensio Anti-Spam plugin for WordPress is a free alternative to Akismet (Automattic's popular anti-spam plugin). Akismet works great, but it's only free for personal sites; for business sites, you need to pony up $5 per month.
I tried Bad Behavior and Growmap Anti Spambot Plugin (GASP), but they both let through 3-10 spam message per day for OptimWise.com. Defensio lets through 0-3 per week.
The Defensio plugin is produced by Websense, a big name in enterprise security. Defensio is free for personal use and for small businesses with up to 5 employees. Learn more on the Defensio for WordPress page.