Take a Quick Screenshot of One Window.
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.
Saved as "Picture 1" on your Desktop.
Look for a file named something like "Picture 1" on your desktop. In Leopard and Tiger, the picture will be saved as a PNG file. Panther saves the image as a PDF.
Even better, your window is captured with full transparency. That means when you open the resulting PNG file in Photoshop, you've retained your rounded corners, and any transparent elements. Terrific for documenting Macintosh software or techniques.
Examples of What You Can Grab.
Grab menu items:
Grab dashboard widgets:
Grab icons from the Desktop:
The results of these grabs are all nicely cropped PNG files that maintain the transparency of the windows or elements. Perfect for documentation. Give it a try.