[wsnipex]

"sudo service xbmc stop" will stop Xbmc.

But since you want to use external applications you should enable a window manager.
uncomment this in the init script and make sure you have fluxbox installed.

Code:

#   exec su -c "xinit /usr/bin/fluxbox :0" $USER

After a reboot, you should be able to switch Xbmc to window mode(\ key) and run any program you want.