If you would like to add CAPTCHA to your website then reCAPTCHA is a nice choice for you. Its very easy to use and the best part is that once you register on their website you can use the same CAPTCHA code across multiple websites thereby saving you the time and effort.
First you will have to register on their website. They don’t ask a lot of questions. Just your email address, username, password and domain. Once you have registered on the website you can view the different plugins provided there. You can also go through the documentation online.
You just have to upload the recaptchalib.php, reference it in your code and away you go. Nothing to it.
Hope the above helped