I had a listing a while back for some key commands in Linux. Since my recent move to Ubuntu 6.10 I have discovered that Ctrl-Alt-Esc does not bring up the trusty old skull and crossed bones. A listener tip on the Linux Reality podcast pointed out this little trick for force quitting an uncooperative application/program:
open a terminal and type the following and press enter: xkill
Apparently this changes your cursor into the familiar skull and crossed bones. Then simply click on the window of the application/program that you wish to force quit or kill.