built into the Mac OS:
To take a picture of the whole screen, press Command-Shift-3.
To take a picture of part of the screen, press Command-Shift-4, then drag to select the area you want in the picture.
To take a picture of a window, the menu bar, the Dock, or other area, press Command-Shift-4, then press the Space bar. Move the pointer over the area you want so that it's highlighted, then click. If you decide you want to drag to select the area, press the Space bar again.
If you press Command-Shift-4 and decide you don't want to take the screen shot, press the Escape key.
Screen shots are saved as files on the desktop. If you want to put the screen shot in the Clipboard, rather than create a file, hold down the Control key when you press the other keys. You can then paste the picture into a document.