Etherpad is a web-based collaborative real-time editor, allowing authors to simultaneously edit a text document, and see all of the participants' edits in real-time, with the ability to display each author's text in their own color. EtherPad is an open source project.
You can quickly set up shared documents and it is fast enough that you can see what others are typing as they're typing it.
One possible negative is that the documents ("pads") aren't protected by a password, so anyone with a link to them can edit them. People immediately compare Etherpad to Google Docs. EtherPad is probably better to use than Google Docs if you want to support anonymity and/or allowing people without Google accounts to participate.
Anyone can create a new collaborative "pad" and each pad will have its own URL. Anyone who has the URL can edit the pad and participate in the associated chats. Each articipant is identified by a color and a name. The software auto-saves the document at regular, short intervals, but participants can permanently save specific versions ("checkpoints") at any time. A "time slider" feature allows anyone to explore the history of the pad. The document can be downloaded in plain text, HTML, Open Document, Microsoft Word, or PDF format. There is also a chat box in the sidebar to allow meta communication.
Google Docs does not do real-time collaborative text editing, so it takes about 5 to 15 seconds for a change you make to show up on other people's screens. That doesn't sound very slow but as the Etherpad creators say, "Imagine if that was true for white boards or telephones." EtherPad (as in ethernet cable) does it in real time which might be important in working as a group.
Etherpad was first launched in November 2008 (as EtherPad) and the software was acquired by Google in December 2009 and released as open source later that month.