A cool little trick when your logged in at the terminal level with an X windows session running e.g Gnome, KDE, XFCE, etc and want the program to start in X.
I would use it to play tricks or open a URL for someone to look at.
To direct a GUI program to start in X start the command with “DISPLAY=:0” and append the command you want to run afterwards. For example to open a web site using Firefox to my site the command would look like this:
DISPLAY=:0 firefox "www.dannytsang.co.uk"