NullifyNetwork

http://me.sphere.pl/indexen.htm

MoonEdit is a multi user text editor, it lets two (or more) people simultaneously edit a single file, with multiple cursors, entry points and near real time collaboration.

I've been waiting for something like this for a while, after posting about it first, then a year or two later seeing SubEthaEdit for the Mac.  Hopefully someone (http://docsynch.sourceforge.net/) will come up with a plugin for Visual Studio.

(Links curtesy of ntk.net)