for ExpressionEngine 2.x
reCAPTCHA is an extension that replaces ExpressionEngine's built-in CAPTCHA with the reCAPTCHA system on member registration and comment forms.
If you don’t already have your site keys, you can get them here. Tick their "Enable this key on all domains (global key)" checkbox if you'll be using multiple domains for development or MSM sites.
recaptcha folder to system/expressionengine/third_party
Add-Ons > Extensions