Free and not too overcomplicated – exactly what I need for the registration form

A protection of the registration form is always a trade-off to protect the blog from spammers and not to exile those who want to register.
Google Captcha is very powerful but may be really annoying, so that a new user will just change his mind and close the browser window.

This one is simple enough not to overload the users but still to stop most of spammers.
And those who go through will not be able to harm a lot, since I protect comments with advanced Google captcha. In this sense I believe to have found the optimal trade-off.

@mike Challis
Keep up the good work, I have donated you a couple of bucks! ??