Using MacOS X 10.6, is there a way to "switch users" in a window? I'm looking for something like xnest.
You can run single applications as another user using
sudo -u shortname /path/to/Application.app/Contents/MacOS/binaryname (note that it requires the executable, not the application bundle!
open fails to start applications as a different user).
A GUI way to do this might be Pseudo, but I haven't tested it since a few years ago
You can also use Fast User Switching to run an application as another user with that user's preferences and data.