I learned about the site when I started my work on reCAPTCHA. It's the same service, but it pays rewards in different currencies. Many prefer to receive money in dollars. The rate for solving 1000 CAPTCHAs is very low — only 80 cents. They pay more at night, but that really harms your eyesight. You can choose between the regular CAPTCHA and reCAPTCHA. Personally, I prefer working with images rather than typing letters and numbers. So far, I've only lasted half an hour, but now I'm thinking maybe it's worth trying again.
