Here’s a trick I rely on all the time. The keyboard shortcut to take a screen capture of a specific, isolated, window in Mac OS X:
Command+Shift+4, then tap the Spacebar.
Most designers know the Command-Shift-4 shortcut which gives you a small crosshair you can use to crop in on a specific part of your window. Tapping the Spacebar afterwards switches to a camera icon that takes a cropped picture of any window you click on.
(Via CreativeTechs Tips.)