We've updated the OS X Startup Keyboard Shortcuts article to include the use of the Recovery HD that is part of Lion and Mountain Lion. In addition, the document now includes links to articles that explain how many of the startup shortcuts are actually used.
Courtesy of Coyote Moon, Inc.
For example, instead of just telling you that holding down the shift key will start your Mac in Safe Mode, we now include links to articles that tell you how to make use of Safe Mode.
Even if you're familiar with many startup shortcuts, stop by for a little refresher course: