Ways to kill processes in *nix-based systems.
pid = process identification
(courtesy of Liz redcountess)
$ps aux | grep
killall <program name>
Some window managers (such as blackbox and fluxbox) have menu entries to kill the window.
(courtesy of enomisy)
kill sends a Unix signal to the process; if it doesn't register a handler, that kills the process. You can't register a handler for signal 9, which means "kill with extreme prejudice".
xkill forcibly throws a connection off the X server.
(courtesy of ciphergoth)