OSX Screen Capture Hints

The last one is useful because you can grab a specific screen area and past it directly into a graphics app (Photoshop) or an email message. When pasting the program creates tiff format image. You can also tap the spacebar to invoke the camera cursor. As you move the camera cursor around the screen windows will highlight - click to capture, places grabbed window on clipboard. This still works in OSX 10.5 (Leopard) but I haven't tested it in anything more modern.

Update

There's an application called Grab that lurks in Applications/Utilities that will do this sort of thing too.