The Grab utility for the Mac (Applications > Utilities > Grab) is pretty good for grabbing screen shots. It does have some short comings though: the format of the output is TIFF, and it doesn't work on the dashboard or for icons.

There are several built-in commands that get around this. The output is PNG, and they work on the dashboard. The only problem is that the commands are so arcane, they are nearly impossible to remember. So as memory is not needed, here are the commands:

Command + Shift + 3 -- This will capture the entire screen and save a PNG on the Desktop. If you have multiple monitors, it will capture a separate image for each screen.

Command + Shift + 4 -- The cursor will change to be big crosshair. Click and drag to define the area of which you want an image.

Command + Shift + 4, then spacebar -- As above, the cursor will become a crosshair. Once this happens, hit the space bar and the cursor becomes a camera. You can can then click on any window, menu, menubar, or icon to save an image of it.

For all of the above commands, you can press the Control key during the initial key presses. If you do this, instead of saving the image to a file on the desktop, the image will be copied to the clipboard.