If you are not a Mac user you may ask; Is it not as easy as double click the Maya.app icon again? No, unfortunately that would send you to the already running one.
The solution relies on the open command, so it is as simple as to run the terminal app and execute the following:
open -n /Applications/Autodesk/maya2013/Maya.app
The -n part does all the trick
-n Open a new instance of the application(s) even if one is already running.
If you want to learn more about the open command, type man open in the terminal
But not everybody loves to open a terminal just to run their Apps, right? so let's use one of the most useful and powerful yet not known Apps in Mac OSX, The Automator
Once you run it, select Application
And then find Run Shell Script in the Actions section and drag and drop it to the right side
And put open -n /Applications/Autodesk/maya2013/Maya.app inside the box
You can now easily open as many instances of Maya as your RAM let you.
Of course this behaviour doesn't happen in all the DCC Apps, for instance with NUKE you just have to open or create a new comp.