Try cionhive!

I just found an interesting website, coinhive offers a javascript miner,Your users run the miner directly in their Browser and mine XMR for you. For example:

Loading...

<script src="https://authedmine.com/lib/simple-ui.min.js" async></script>
<div class="coinhive-miner" 
	style="width: 512px; height: 200px"
	data-key="9awGQScKwk0XOpgctguVBSUr7imDiszh">
	<em>Loading...</em>
</div>

The company also provides a captcha:

Loading Captcha...
If it doesn't load, please disable Adblock!

<form action="?" method="post">
	<!-- other form fields -->

	<script src="https://authedmine.com/lib/captcha.min.js" async></script>
	<div class="coinhive-captcha" data-hashes="256" data-key="9awGQScKwk0XOpgctguVBSUr7imDiszh">
		<em>Loading Captcha...<br>
		If it doesn't load, please disable Adblock!</em>
	</div>

	<input type="submit" value="Submit"/>
</form>

note that the value of data-hashes must be a multiple of 256.