For the last while I've been using a combination of the reCAPTCHA service with the reCAPTCHA module, via the CAPTCHA module, and the http:BL module in an effort to reduce the volume of spam on my site, and they've been working really quite well since I upgraded the site to Drupal 7. Tonight I'm adding another item to the mix - the BOTCHA module that adds some additional tricks to help prevent spam. I'll see how it goes.

Update: After monitoring it for a little while, BOTCHA and CAPTCHA were blocking the exact same messages & actions, so I've disabled CAPTCHA entirely and will see how it goes for a while.