If any successful site were to measure how many real users didn't signup because a stupid captcha was thrown in front of them during signup process, I am sure the number would be astonishing.
Captcha is double edged sword while it prevents automated signups by bots it also pisses of lots of genuine users.
Visual captcha is nice alternative to captcha but again it is a another mess. There are tons of captcha alternatives listed
here. But none of them seem to work seamlessly.
What i think is site or forum should allow users to login via their facebook or google or hotmail account and then let big boys handle the bots.
My two cents