Google has created invisible reCAPTCHA. Only robots and suspicious users will become required to check a "I'm not a robot" box. All others will be detected and not required to enter or check off on CAPTCHA hence, invisible.
Google's reCAPTCHA is a free service service that is supposed to protect your website from abuse and spam. This service builds a database each time CAPTCHA is required. In order to use this service you must have an existing Google account or sign up for one.
Visually impaired individuals will be prompted with an audio CAPTCHA, if you enable this option. This invisible reCAPTCHA widget can be used on your blog, forum, sign up form, etc. Also, you will need to sign up for an Advanced Programming Interface key pair, for your website domain.