User-friendly spam protection.
Honeypot helps prevent spam bots from autocompleting forms. CAPTCHA asks a question to determine if the user is a bot.
Mollom is content moderation service that not only uses posted content, but also the past activity and reputation of the poster to detect spam.
Some modules that extend CAPTCHA functionality: Image CAPTCHA Refresh, reCAPTCHA, Captcha Riddler and Draggable Captcha.
CAPTCHA adds a challenge-response test to determine if form submitter is a human.