Writing is a major skill that all English language teachers will at some time have to teach. The internet is a boon for writing teachers! It offers synchronous (at the same time)  or asynchronous (at different times) writing platforms that allow the teacher to add, comment, correct in real time.

The two best of these are the Ethernet copy - Google Docs and Pirate Pad (also Etherpad)  [outlined on this post]

1. Google Docs.  

The leader in online collaboration, google uses a "Cloud" process so your documents are stored and reachable by you and others wherever you may be. You can use google docs with a gmail account or a google account (use your non gmail email address).Take a tour here.

With google docs, all your students can have access to a document or "create/build" a document. It is a complete word processing suite - very powerful.  Students can leave messages and see old/new versions. of documents. There is online chat so that members of the document "team" can chat at the same time as they work. Cool stuff and students can work together either at school or at home, on their projects/documents.

2.  Pirate Pad

Pirate pad is a clone of the old Ethernet application. Also see iEtherpad. It is very simple and easy to use. Simply start making your document quickly and there is a very flat learning curve. Like google docs, but even in a better way, you can see old versions of the document. Also, it has chat for those working collaboratively. All in all, I recommend it if you don't need a lot of formatting "stuff" and are just working on one simple document, project. Usually the case in our teaching....  You can open and make a page on the fly by just adding the doc name at the end of the url <http://ietherpad.com/mydocumentname >

Here are all the Best Online Websites for Teaching Writing that I recommend!