Windows/Mac/Linux: Gobby is a free, cross-platform collaboration tool that makes it easy to collaborate on text documents over the internet with anyone. Every time you start a session with Gobby, you choose a highlight colour that Gobby uses to indicate which sections of the text are being edited by which users. Gobby works like a charm for any text document, but with support for syntax highlighting, it really shines for collaboratively editing source code. It's not quite as simple to get started with as something like Google Docs—for example, you'll need to be able to send other collaborators your IP address—but it's an incredible tool in its own right. In fact, it's the collaboration tool that Ubuntu founder Mark Shuttleworth told us he uses.