I have been experimenting again with Hackpad recently. Since I last looked Hackpad seems to have been acquired by DropBox. But I note that Hackpad has some useful features for teachers of computer science - not least that any code you paste in has line numbers by it.
Here is a little bit of code I'm going to get my GCSE computer scientists to improve so that you can see what it looks like:
<script src="https://hackpad.com/x8vxgxb6G6u.js"></script><noscript><div>View <a href="https://hackpad.com/x8vxgxb6G6u">Debugging and alpha testing...</a> on Hackpad.</div></noscript>
Addendum- Hackpad has now be acquired by DropBox, so the above is gobbledygook!