The Honeypot Captcha: It's so simple! Place an input on the form that isn't visible to the human user, but is visible to a bot that isn't scrutinizing css stylings. When the form submits, you know that a bot has filled out the form is the honeyput input contains data.
The big question: why did it take so long for people to think this up? The next big question: why are we still using capchas? The final question: why is it that Google's capchas are the worst damned capchas I've very had the misfortune to use?