Well, ReCaptcha will do the job and there are many tutorials on it online.
However, you have to write a correct "def create" (create method) in your controller that will pass whatever is in your form plus validate Recaptcha at the same time. Then it will work nicely.
There was one little problem with it. After I inserted ReCaptcha into my form, the form validation stopped working. However, it can be fixed with an easy code inserted into the model file:
after_validation :on => :create
(:create = is the "def create" method in your controller). It will force the form to validate the form first and then validate Recaptcha.
