If your system is utterly unresponsive, you can press Alt+SysRq+K (the sysrq key should be the same as the Print key) to Kill the X display server and all its child processes.
This will throw you back to the GDM login screen, killing all applications currently running on TTY7, which is the one where X is running.
→ More information on Magic SysRq Keys at Wikipedia.
Of course you will lose all unsaved data, and you won't know what process went awry, but it will respond even if some process prohibits you from doing everything else, the magic sysqr keys have highest priority.
If your system is still responsive enough to do what Javier Rivera suggested, this is obviously preferable. Type in
ps aux on the command prompt you'll get through Ctrl+Alt+F1 to get a dump of all the processes currently running. You can use Shift+Page Up to scroll though lengthy output.