Really Simple CAPTCHA is a CAPTCHA module intended to be called from other plugins. It is originally created for my Contact Form 7 plugin.
I have to give a poor rating to this plugin as it is supposed to work with Contact Form 7, but in many cases it conflicts with it, generating the perpetual spinning arrow problem. Plugin author has been informed of the issue through the support forum but insisted on giving the same answer (check for conflicts with theme or other plugins). I really hope this issue will be resolved; in that case, I'll come back and give it a better review.