I recently started to poke around with drupal for the content of a couple of sites I host.
Thats when I stumbled upon this nice take on capthas.
Traditionally a captcha consists of random letters and words, that are more or less readable, to stop spamming scripts. This is a problem since people also have a somewhat hard time reading garbled random characters.
Recaptcha uses a different approach, it lets users decode scanned words from books that OCR software can’t handle. This means that what you see are actual words which is easier for humans to understand, and that it should be sufficiently hard to decode by software. Also you help digitalize books. It is really a win-win situation!
An example of it in use can be seen below, or in the comment form on this blog.