I spotted this trick on macosxhints.com: An easy way to run multiple instances of any program
Apple’s Developer Resources has a copy of the man page for open and explains it like this:
-n Open a new instance of the application(s) even if one is already running.
So what can you do with it, and why?
I had no real use for it until this morning when I wanted to test the CPU load on Safari of a web site, without having to close all my windows and their tabs. So I fired up the terminal and did;
open -n /Applications/Safari.app
… and then there were two Safari icons in my Dock!
The MacOSXHints article warns that there is some danger having multiple copies of an application open, as they will all be trying to read/write to shared files like preferences.
Let me know if you have any cool uses for it!