reCAPTCHA

This seems like a powerful (and free) CAPTCHA service. I notice Ticketmaster is using reCAPTCHA.

An interesting side note: this software is simultaneously using the human interaction to help digitize books! Wild.

“reCAPTCHA improves the process of digitizing books by sending words that cannot be read by computers to the Web in the form of CAPTCHAs for humans to decipher. More specifically, each word that cannot be read correctly by OCR is placed on an image and used as a CAPTCHA. This is possible because most OCR programs alert you when a word cannot be read correctly.”

http://recaptcha.net/

reCAPTCHA helps prevent automated abuse of your site (such as comment spam or bogus registrations) by using a CAPTCHA to ensure that only humans perform certain actions.

• It's Free! Yep, reCAPTCHA is free. The only reason we might charge is if you require special services from us.
• It's Useful. Why waste the effort of your users? By using reCAPTCHA instead of other CAPTCHA implementations, you are helping to digitize books.
• It's Easy. reCAPTCHA is a Web service. As such, adopting it is as simple as adding 4 lines of code on your site. For many applications and programming languages such as Wordpress and PHP we also have easy-to-install plugins available. We generate and check the distorted images, so you don't need to run costly image generation programs.
• It's Accessible. Most other implementations of CAPTCHAs block visually impaired individuals, who cannot read images of distorted text. reCAPTCHA, on the other hand, has an audio test that allows blind people to freely navigate your site.
• It's Secure. reCAPTCHA is run by the original creators of CAPTCHA and has the highest security standards. Many other implementations of CAPTCHAs can be easily broken.