Etherpad
Etherpad is an application that is based off java that allows multiple people at the same time to update multiple documents. Now I realize you might be thinking google docs does something like this. It is close but it is not the same. There is a delay in time that it updates on other persons screen. Also there is no support for syntax highlighting for web development.
It is a free service and they offer a paid solution if you want to host it on your internal network. My company I work for just bought some user licenses for it. So bookmark that link cause if you need to show someone else a document and be able to edit it at the same time, then etherpad is your solution.