A friend and client, Luis von Ahn, gave a great TEDx Talk at Carnegie Mellon University in Pittsburgh. Luis runs through the creation of the captcha, his project reCAPTCHA, and his new language learning project, Duolingo.
One of my favourite things about working with Luis, is that one of his lead developers and co-founder of Duolingo is actually blessed with the name Severin Hacker.
I use reCaptcha on all my forms, and it helps me to get rid of spammails.