Zoho enables simultaneous editing and collaborative discussion of a document.
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After creating or uploading a document, you can invite other people to edit it or simply view it. If two parties open a document at the same time, Google Docs makes a best attempt to merge differences between the two, and tracks every single person's revisions in a very detailed fashion. The differences are only available in a separate window, though; they're not actually shown in-line as they happen. You can send e-mails back and forth to your collaborators, but there's no real-time chat system -- apart from perhaps Google Talk, although you have to invoke that manually between you and the other person.
My favorite feature with Google Docs: offline editing via Google Gears or the Chrome browser. Document check-in and -out works properly with it, as it ought to, so you can later merge multiple versions of work done offline by multiple parties. Finally, Google Docs has accurate enough document conversion for most basic texts that you can always compose offline in your word processor of choice, then upload into Docs to do the actual sharing and editing.
A strong competitor for Google Docs, Zoho Writer lets you invite collaborators to either examine as read-only or read and work on documents you've prepared. Read-only mode presents the document as a static Web page -- more or less the same view you get when you simply publish a Zoho document to the world -- but for full read/write access, the other user must have a Zoho account. Fortunately it isn't hard to set one up.
The best thing about Zoho is how it handles simultaneous editing of a document. If you edit a document that someone else has open, everyone involved gets notified of this, and any parts of the document being edited by the other person are marked as read only. Changes to the document appear in real-time, too. You can also chat in real time with other people working on the same documents, so you can actively discuss changes as they're being put into effect.
DabbleBoard's almost exactly what it sounds like: a collaborative whiteboard application. You and your partners can draw freehand, insert pictures and text annotations, and chat back and forth with each other. It's all Flash-driven, so it's not limited by the constraints of dynamic HTML or the browser itself.
One can get started with a trial DabbleBoard canvas without even so much as having to log in. Documents can be imported and pasted into the canvas, and canvases can also span multiple pages -- hugely useful if you want to do multipart presentations that don't fit comfortably onto one screen. I also liked how objects on the canvas can be manipulated very precisely by typing in coordinates or otherwise manually specifying properties, instead of just being dragged around.
Drawing freehand on a DabbleBoard canvas works one of two ways: you can either just draw any shape (sort of like scribbling with a marker), or you can elect to have the canvas guess what kind of shape you're trying to make (circle, square, triangle, etc.). The guessing algorithm's pretty accurate, although sometimes drawing shapes inside other shapes got it confused.