The best replacement for captcha is akismet.

The best alternatives to captcha are rated as following:
★★★★★  akismet
★★★★★  honeypot
★★★★  mollom
★★★★  recaptcha
★★★  gotcha
★★★  turing test
★  csrf
★  bots
  decaptcher

Your alternative for captcha is not listed? Suggest!
Speaking about captcha, one probably shouldn't forget about contact form 7.

Other search terms related to captcha and their ratings are:
★★★★★ contact form 7
★ formmail
★ jquery validation
★ recaptcha
You have in mind a companion for captcha and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of captcha in all years. Click on graph for more.
captcha
akismet
honeypot