Celtx is a great and all for writing stories, but when it comes to online collaborative writing our team uses Gobby. Gobby allows multiple people to work on the same text file at the same time. You don't have to designate one person as the scribe. You don't have to take turns. You can type where ever you want when ever you want, and any text you insert will be highlighted with your user color, so you always know who wrote what.
Gobby was created with programmers in mind, so it features line numbering, auto indenting, and syntax highlighting. As writers we ignore those features except for the line numbering. Our text documents can get quite long and it's easy to say "look at line 2053," because they can press Ctrl-I and enter that line number in to zip them to the same line you're looking at.
The user highlighting has also been useful for us to tag sections that need attention with bright red or some other obnoxious color that you can't possibly miss. We just open up a new instance of Gobby, log in as another user, change the highlight color for that user to something obnoxious, and off you go. One of the text documents we are working on with Gobby is the outline for our previously top secret Lumaglyph project (I'll make the announcement for that in a week or two) that is currently over 2000 lines long. To give you an idea about how long that is, a paragraph still counts as one line, and our outline would be 72 pages long if we printed it out. So needless to say that the line numbering and creating multiple users with loud highlighting colors has been a godsend in helping us work with gigantic text documents.
Even though Gobby has no rich text support or undo (be sure to save often!), it has still been a great tool in helping us collaborate in real time on the same document even though we are all in different states. And even if we were in the same room we'd still use Gobby; it's simply that useful.
For those that are curious, there are alternatives to Gobby out there that we have looked at. EtherPad is a web app that works a lot like Gobby, but if you are concerned about having your work unencrypted on their server you may want to use something else, but EtherPad does have undo! SubEthaEdit has some nice features like showing what area of the document a user is currently looking at on the scroll bar, but unfortunately it's OSX only and costs money too. Mozilla Labs is working on a new web based editor called Bespin that has a really slick and promising interface, and a lot of neat features that are definitely geared for programmers. That might be a project to keep an eye on as it's still very experimental at the moment. MoonEdit is also worth a mention (Windows and Linux only, free for non-commercial use).
But out of all the alternatives out there, Gobby has worked the best for us. It works on Windows, Linux and OSX (however getting it to work in OSX is a huge pain), Gobby is encrypted over the internet, password authentication is supported, and it's free and open source.
The Triad of Collaborative Writing
As with all real-time text editors, voice chat and secure online storage for your work is still a necessity. We use Skype for voice chat and our own MediaWiki server for storing and organizing information about all our projects. The wiki has been, and still is, the keystone of our collaborative work. The best way to explain how we use the wiki for this particular project is that it's a series bible, filled full of backstory, character profiles, a milieu encyclopedia, and of course the outline and scripts, all cross linked to each other and full of reference and original images, just like Wikipedia (in fact Wikipedia runs on MediaWiki). Our Gobby documents are done in wiki code so that we can copy and paste them right into the wiki so these documents are easy to find and read.
We originally used Gobby for taking meeting notes during our brainstorming sessions, which we would then copy and paste into a new meeting notes page in the wiki to be sorted sometime later (usually weeks later). When we did finally get around to that, we'd go through all our meeting notes and sort them into the wiki, making new wiki pages as necessary. It was pretty tedious. Now, this happened during the initial brainstorming phase of the project, so everything was kind of a mess in terms of categorization anyway. But when we came to the point where our main focus was the outline, we were working in Gobby pretty much most of the time, and usually with just 2 documents in the Gobby session: one document for our outline and then the other for our random notes, which we could then sort directly into the outline or into the various pages on the wiki. With those smaller non-outline related notes we'd often make those changes directly in the wiki. Once we started doing that process our lives became easier and we spent more and more of our time just writing and coming up with ideas. We save the Gobby session locally on our computers so we can keep all the user highlighting until we are ready to clean the slate again. And we've made an aggreement to not edit the outline on the wiki because of all this.
Until Celtx—or even better MediaWiki, once they've added a few more dream features of ours—ever gets around to implementing real-time collaboration, Gobby is the best solution we've found as writers working together online.