What is the equivalent of 'Control-Alt-Delete'?

System Monitor shows you an overview of running applications (under the Processes tab) and allows you to end them by right-clicking on the name and selecting the respective context menu item. You can set up keyboard shortcuts in the Keyboard settings (Shortcuts tab), so you can bind Ctrl+Alt+Del to gnome-system-monitor, the command to start System Monitor.

Another command you might be interested in is xkill. This is traditionally bound to Ctrl+Alt+Esc (not sure if by default) and allows you to click on a misbehaving window to close it.

Both of these might not work if your entire system hangs. If that happens, there are two things you can do. The first has to be enabled in advance when your system doesn't hang yet (you could do it right now): open Keyboard Layout settings (I believe this is merged into Keyboard in Ubuntu 12.04), then click Options. One of the options is Key sequence to kill the X server, you can click that run gsettings get org.gnome.desktop.input-sources xkb-options for recent versions of Ubuntu (according to this answer). If you have done that and your system later hangs, you can press Ctrl+Alt+Backspace, which will effectively bring you back to the login screen.

If even that doesn't work, the last thing you can do is ensuring a sane shutdown (i.e. not pressing and holding the power button). This one is a bit hard to remember, but it involves pressing and holding Alt+SysRq (system request, often same key as PrtScr print screen) and then press in order R, E, I, S, U, B (a mnemonic is Reboot Even If System Utterly Broken). Wait a second between each presses, because each letter is a different kernel action leading to a "graceful" reset.

To stay in the spirit of magical shortcuts, here are some :

• ctrl+alt+backspace combination (disabled on default on 11.10) that can restart the GUI.

• ctrl+alt+F2 to F6, that will display a console from which you could login and then eventually kill the stucked application. Once you killed it you can return to the GUI by pressing alt-f7. Killing an application by name can be done by using this command:

  sudo killall <name-of-the-application>
  

  If this doesn't work, use the -9 flag to kill it with überforce.

  sudo killall -9 <name-of-the-application>
  

• IN CASE OF EMERGENCY : use the Magic SysReq Key to directly "speak" to the kernel.

you could have a short command for opening a terminal, I have F4. When you need to force an application to quit just open a terminal and type xkill and then click in the window of the application that has crashed.

GNOME-shell

1. System --> Preferences --> keyboard shortcuts (gnome-keybinding-properties)

2. go down to the Desktop list, click on Run a terminal then hold your short command.

   the short command should be visible in the line on the right.

To kill a process

1. Open a terminal. (Or just use Alt+F2.)
2. Type: xkill
3. Click in the window that has crashed.

Having a short command for opening a terminal is a good idea in general because you can do other things as well. (htop, system monitor, rebooting, etc.)