Disclaimer: I actually copied this text verbatim from my blog. I always hate it when people post their own blog entries on Digg or forums, but I thought that this tip was worth sharing.
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Reason #264 why you'd love Ubuntu: quick application termination
Ubuntu allows you to quickly terminate unresponsive applications with the press of a button. Just create a shortcut to 'killall
processname' and you're all set up. Popular uses of this little feature include killing Firefox ('killall firefox-bin') and making all command line media players shut up ('killall mplayer').
If you don't know the process name of the application you want to set up a kill switch for, look for it in GNOME's task manager. You can find it in the System menu or if you prefer the quick and dirty method, run 'gnome-system-monitor'.