Actually, I've written this post before (quite a few months ago) but then deleted it because it didn't work. In the mean time, I found out why, so I decided to post it again. I find pressing Control + Alt + Delete to open up System Monitor to be very useful, especially for Windows users who are used to it and may actually press this quite a few times before realizing it doesn't do anything or what it does in Windows. Using System Monitor, you can preview all running processes, end or kill them, see how much CPU a process is using, CPU + memory + network history, available disk space and even change how much CPU a process should use.

There are 2 ways of changing Ctrl + Alt + Delete behavior to open System Monitor: one if you are running Compiz, and one for Metacity, only to use if you are not running Compiz.

Step 1: This is actually very important: go to System > Preferences > Keyboard shortcurs and search for any command which already has Ctrl + Alt + Delete set as a shortcut and change it's shortcut. If you don't change this, it will not work! If you want to use the "Del" ("keypad .") button instead of "Delete", search for that and also make sure there's no shorcut set for it.

Once you complete step 1, we can proceed.