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.