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From The Official CAPTCHA Site

reCAPTCHA is our officially recommended CAPTCHA implementation.

Test Drive a CAPTCHA

• reCAPTCHA. Stop spam and help digitize books at the same time! The words shown come directly from old books that are being digitized.
• SQUIGL-PIX. Our newest CAPTCHA!
• ESP-PIX. A CAPTCHA script that's close to our hearts. Instead of typing letters, you authenticate yourself as a human by recognizing what object is common in a set of images. This was the first example of a CAPTCHA based on image recognition.

Read more: http://recaptcha.net/security.html