I'm generally annoyed when I visit a site and I'm faced with a CAPTCHA to either register or post a comment. I'm well aware that computer-aided spam is a huge problem on the Internet but it always seemed to me that there either had to be a better way to solve the problem... or maybe there was a way to utilize my wasted time better. The only problem is I'm obviously not smart enough to come up with one.
Enter reCAPTCHA. Feel free to read that link in detail, but the basics are now CAPTCHA challenges can serve a common good, digitizing hard-to-OCR books.
Even cooler is reCAPTCHA's Mailhide service. With Mailhide you can feel much safer providing your e-mail address on a web site as it will be locked behind a CAPTCHA challenge. Check out the right side of the main page to see how this works in practice.