One of my favourite sites from 2009 was an amazing tool called Etherpad. It allowed multiple users to collaborate within the same writing space from anywhere in the world at exactly the same time. It was brilliant for collaborative writing, collating ideas during a training session/lesson and pooling the knowledge of experts via the internet.
Sadly, after being bought out by Google, Etherpad is closing down in the Spring. But they have now made the code open-source, and that means that other sites are now springing up that are running the same software as Etherpad.
One of those, with UK schools in mind, is Primary Pad.
It works in exactly the same way as Etherpad. You can sign up for a private, passworded pad as well if you wish.
I would expect more and more Etherpad clones to appear as the deadline for etherpads closure gets closer. You can check on the Etherpad page on Wikipedia to keep tabs on all the latest ones.