Andreas Cellarius was a scholar of the 17th Century who produced one of the most famous cosmological atlases of all time, Harmonia Macrocosmica, featuring 29 beautiful plates (large, high-quality scans), illustrating various aspects of the Universe as understood by the Western science of his time. It's impossible to pick favorites among them, but here are three examples: Phases of the Moon, Sizes of the Celestial Bodies and Stars and Constellations of the Northern Sky.
Several months ago, Bill Rankin of Radical Cartography (previously and previouslier) created an astronomical calendar of events for New Haven, Connecticut, where he lives, featuring all of the inexorable rhythms of the Solar System in one handy PNG file. Now you can create such a calendar for any location on the planet, with information as basic as the hours of daylight or as esoteric as the tilt of Saturn's rings, all lovingly rendered in soothing translucent pastels. [more inside]
Zoom around the Milky Way at different wavelengths with Chromoscope: X-Ray, Visible, Hydrogen α, Far-IR, Microwave, Radio. (You can also download it.)
3D Starmaps by Winchell Chung. (I knew him for his game illustrations before I ever knew about his starmaps.) The site contains lots of information about how to make 2D/3D starmaps from standard star tables, a nice selection of pre-existing maps and one of the best listings of 3D starmap software around.
The U.S. Naval Observatory Library features high-res scans of images from antique books dealing with astronomy and navigation. Wallpapers, ahoy!
World Sunlight Map. A neat little map showing the encroaching blob of darkness as parts of the world slip in and out of nighttime.
3-D Maps of Nearby Space "The first detailed map of space within about 1,000 light years of Earth places the solar system in the middle of a large hole that pierces the plane of the galaxy...The new map, produced by University of California, Berkeley, and French astronomers, alters the reigning view of the solar neighborhood." (one view|another view|links to bigger images)
Celestial Atlases are perhaps some of the most beautiful scientific books ever published, capturing the mystery and the grandeur of the heavens, and rife with beautiful and often intimidating interpretations of the constellations. Out Of This World has been my favorite website since the dawning of time, and one I go back to over and over again even though it never changes. The period from 1603 to 1801 produced the most beautiful star maps, and you don't have to know a thing about astronomy to appreciate how heavenly these are.