reCaptcha and Save a Book
Web these days is full of spam-bots, malicious crawlers and other "e-pests" that post junk-advertising all over the Net. These parasites give the most headache to blogs and social sites, where moderators want to free-up comments to general audience ("Two-Way Communication", eh?), but have no desire to promote illegal Viagra sales.
Captcha, or Completely Automated Public Turing test to tell Computers and Humans Apart, is one of the most effective tools to fight evil machines with. Unfortunately, not all Captcha is equal. Many Captcha systems are vulnerable and have been hacked, rendered useless against all but the most primitive spamming.
reCAPTCHA is a service from Carnegie Mellon University. This service is a prime example of blending pleasant with useful in a very Web 2.0-ish way. reCAPTCHA provides a free, high-quality protection and at the same time helps digitize old books. Every time you use reCaptcha, you help digitize one word of a book that was written before the digital age. How much cooler can it get? Well, reCaptcha also provides enhanced accessibility through audio-Captchas. That one is not easy to "code" yourself, and I don't know of any other free service that offers it.
Great job, guys!