One of the (many!) reasons I’m a happy OS X user is that the operating system itself has a swathe of productivity enhancements built in right out of the box. Most people know about the Grab application for making screen shots, but there are some shortcut keys to make the screen shots without the need to start up a separate application that are much less well known.
Command-Shift-3 (note: that’s 3 folks, not F3!) saves a picture of your entire screen right to the desktop. That means, whatever is being displayed on your screen goes into the file, even if you are running Dashboard Widgets, Expose, Spaces or even Quick Look at the time!
Command-Shift-4 turns your cursor into a cross-hair. Click and drag over an area of the screen, and this is captured into a picture on your desktop. Better yet, if you hit the spacebar before you click, the cursor changes into a camera, and will capture the whole window that you click on (even if a portion of it is hidden behind something else at the time).
Control-Command-Shift-3 and Control-Command-Shift-4 behave almost the same as their non-Control-chord brethren, except that the shots are saved directly to the clip-board for pasting into an image editor instead of being saved to a file.
You can also change the format of the image files from the terminal:
defaults write com.apple.screencapture type pdf killall SystemUIServer
Or you can choose jpg, tiff or png. Once you’ve killed the SystemUIServer you’ll have to wait a few seconds for it to restart, and be able to make more screenshots in the new format.
If these keys don’t work for you, then check the Keyboard Shortcuts tab of the Keyboard & Mouse preferences to see if there is a shortcut conflict or if those shortcuts have just been disabled.