Google Sky: Browse the whole sky
Google Moon: Browse Moon
Google Mars: Browse Mars through Google
Chromoscope: View the Universe in different wavelengths
Astronomy picture of the day: Each day a different image or photograph of our universe is featured
An Atlas of the Universe: Contains 3D maps of the universe zooming in from the galaxy to nearest stars
The Nine Planets Solar System Tour: Information and overview on nine planets of the solar system
NASA/IPAC extra-galactic database (NED): contains information about more than 500,000 extra-galactic objects
SIMBAD: information about stars
International Astronomy Union: The official body responsible for astronomy in the world.
Arxiv: Draft of the recent astronomy and astrophysics articles.
MyStar: A stellar lifecycle game. Good for understanding stellar evolution.
Meteor Watch: An interesting site on meteors and astronomy.