Recently Google have acquired the Re-captcha service, which is one of the very famous AntiSpam tool available in internet. I have tried so many anti bot systems to prevent spam in my forums, but the only best option i found is Re-Captcha.
Re-Captcha displays words from books to fight spam in the registration forms, forums and so on. So far google have not implemented this antibot service in any of its products or services. But the acquiring increases the expectation among web site owners and bloggers that it will surely come up with more seemless api and integration features.
http://recaptcha.net/
