Snaptcha (Simple Non-obtrusive Automated Public Turing test to tell Computers and Humans Apart) is an invisible captcha and will be the last time you will ever have to think about protecting your forms from spam bots.
Snaptcha will automatically add a hidden field to your forms and will validate it on submission. If it determines that a spambot is attempting to submit the form then it will prevent the form from being submitted and will output an error message. You can control the level of security as well as the error message text.
The following forms are supported:
In-depth review: Out with the Spam! Snaptcha for ExpressionEngine
Reviews taken from devot-ee