Shazu is a game that pairs you with a random partner to write sentences one word at a time. Its a pet project built with node.js, redis and websockets. Feedback welcome!

Is there anything similar to this that doesn't require registration?

I'd love to use this for teaching HTML but I can't ask my hundreds of teenage students to all create accounts...

Collaborative isn't a must, anything with syntax highlighting and real-time previews will do.

First of all, registering takes few seconds with twitter/github.

Secondly, it's not a must. Only you can register and share the collaboration URL with your 100s of students.

They can collaborate if they want to, or you can teach them in "Teacher Mode" - registration is not a must!

Hope that clarifies your doubts :)

Plunker has existed for a while now: http://plnkr.co and has its own little micro community. The editor (with real-time collaboration too) is at http://plnkr.co/edit/