You can set Mac OS X to start certain programs when you log in, but sometimes you just want to get into your computer quickly without all that nonsense. With a simple keyboard shortcut, you can.
If you hold the Shift key down just as your computer starts to log you into OS X, it will ignore all your login items. This is really handy if you have a ton of login items (like me) but just need quick access to a browser or something for that session. I could only get this working with automatic login disabled, but it's possible my finger just wasn't fast enough. You have to hit it right as your computer starts to log you in.
Similarly, you can actually disable automatic login for a specific session by hitting the Shift key just as your Mac switches from the white screen to the blue screen as it boots. This is handy if your computer is used primarily by you, but every once in a while by someone else. Weblog Mac OS X Hints notes it's also useful for making sure thieves are able to log in to your computer so your anti-theft app can track it.
Last but not least, if you hold Shift just as you hear the boot chime, your Mac will boot into safe mode, which will disable certain features like file sharing, resource intensive graphics, and more. This is useful if your computer starts having problems. Hit the link to read more.
Temporarily Disable Automatic Login [Mac OS X Hints]