xsu Is an interface for the command `su - username -c command` in Gnome.
When the user executes xsu, he will be prompted for a command, a username 
and a password. If the username and password are correct xsu will execute 
the command   as the entered user. So if the username was root, then the 
command will be executed as root.

WWW: http://xsu.sourceforge.net/