Astronomy Picture of the Day
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
http://antwrp.gsfc.nasa.gov

Hubblesite.org
Home of the Hubble Space Telescope, a NASA Origins mission that seeks answers to fundamental questions about the universe and our place in it.
http://hubblesite.org/

AbsoluteAstronomy.com
General facts and statistical information about planets, moons, constellations, stars, galaxies, and Messier objects.
http://www.absoluteastronomy.com/

Sky and Telescope
Sky & Telescope, the Essential Magazine of Astronomy. News, observing tips, how-to advice, and more!
http://skyandtelescope.com/