Ubuntu - Force close a non-responding window

The xkill command allows you to force close a window or application with your graphical mouse pointer.
Invoke it by opening up a terminal and typing:

xkill

After this your mouse pointer will turn into an X allowing you to force close any window you click on.

The pkill allows you send SIGTERM to processes just by specifying ( part of ) their name:

pkill firefox
 

The traditional kill command requires you to specify / retrieve the process id first:

$ ps -ef | grep firefox
1987 ?        Sl     8:32 /usr/lib/firefox-3.5.4/firefox
 
$ kill -9 1987

The killall command allows you to kill multiple ( all ) processes based on ( part of ) their name

killall firefox