I've used protectwebform.com for a few months but currently I don't satisfied with their captcha and buttons. I've googled a bit, and found these services: http://webspamprotect.com and http://recaptcha.net Anybody using it? Please advice.
In my experience, recaptcha.net works great. It's easy to install, easy to use, and you help digitizing books at the same time.
Thanks for replying. Recaptcha looks great, but I'm confused with a difficult captcha they offer. And each time you have to type 15-18 symbols. I'm not sure that my users will appreciate it. I want a google-like captcha as on this page - webspamprotect.com/signup.php Anyway, thanks for advice. I'll try to install it.