Windows: Everyone knows how to open the task manager: press Control + Alt + Delete and click "Task Manager," right? Still, the entire point of using keyboard shortcuts is to avoid having to use the mouse. Press Control + Shift + Escape to bypass the menu that appears and go directly to the task manager.
Photo remixed from an original by Charles Dyer.
To be fair, on Windows XP systems where the welcome screen is enabled, both shortcuts take you directly to the task manager. On any other Windows XP, Vista, or Windows 7 system, pressing Control + Alt + Delete will bring up a series of options, and the task manager is one of them.
The beauty of Control + Shift + Escape is that you skip the options screen and the task manager simply appears on your screen. That can be a big relief if the whole reason you want to open the task manager is to kill a program that's hogging system resources – system resources your computer would need to display that pesky options menu.