Akismet is installed and doing a phenomenal job. Since Akismet doesn't charge sites that don't make a profit (like my site), I'd like to reduce the use of their resources as much as possible.
What I'm looking for is a "Completely Automated Public Turing test to tell Computers and Humans Apart" (CAPTCHA) plugin for WordPress 2.2. CAPTCHA generates those funny squiggly letters/numbers that machines cannot read, but humans can. A user has to type in the letters/numbers before their comment can be submitted.