reCAPTCHA is a free captcha service provided by Google. It is designed to be easy for humans and difficult for bots.
To use this package you must have a Google account and sign up for the reCAPTCHA API. After installing the package go to System & Settings -> Permissions & Access -> Captcha Setup in the Dashboard, select reCAPTCHA as the active captcha provider and provide your site and secret keys obtained from Google.
Source code for this package is available on Github.