We've shown you how to abort shutdown in Windows before, but reader judacris shows us that it's even easier in OS X — just click on any app in your dock.
If your Mac has a few applications open, it will take a few seconds to shut down since it needs to quit all of them beforehand. If you hit the shut down button accidentally (or change your mind in the middle of shutdown), all you need to do is click on a random dock icon. You'll get a popup notification that "the application cancelled shutdown", and you can continue working without waiting to restart your computer. Simple, but handy!