I've had amazingly good results with a simple "Leave this field blank:" field. Bots seem to fill in everything, particularly if you name the field something like "URL". Combined with strict referrer checking, I've not had a bot get past it yet.
Please don't forget about accessibility here. Captchas are notoriously unusable for many people using screen readers. Simple math problems, or very trivial trivia (I liked the "what color is the sky" question) are much more friendly to vision-impaired users.
