Google reCAPTCHA

HTML

<script src="https://www.google.com/recaptcha/api.js"></script>
<div class="container">
    <div id="recaptcha"></div>
</div>
<div id="email">
Verify captcha to get e-mail
</div>

CSS

.rc-anchor-pt{display:none;}

#recaptcha {
  display: inline-block;
  position: relative;
}

#recaptcha:after {
    content: "";
    display: block;
    position: absolute;
    z-index: 1;
    bottom: 3px;
    right: 5px;
    width: 100px;
    height: 70px;
    background-color: #f9f9f9;
}

JavaScript

function createRecaptcha() {
	grecaptcha.render("recaptcha", {sitekey: "6LcgSAMTAAAAACc2C7rc6HB9ZmEX4SyB0bbAJvTG", theme: "light", callback: test});
}
 createRecaptcha();

function test() {
	// ideally you would do server side verification of the captcha and then the server would return the e-mail
  document.getElementById("email").innerHTML = "[email protected]";
}