RAILhead Design: APOD Grabber is a cool, little Mac OS X app that gives you the ability to quickly and easily view, browse, and download images from NASA’s Astronomy Picture of the Day web site. It also grabs information about the picture and allows you to set the Desktop background too.
Found this excellent image of the night-time Earth today using it. See Earth at Night. You can grab the images as various resolutions but this would make an excellent poster or background.