• Epo's Chronicles (a web comic from the outreach group at Sonoma State - very cool!)
• Sky & Telescope
• Astronomy
• Star Date
• Universe Today
• SPACE.com
• The Astronomy Cafe
• Earth and Sky

• the universe adventure (a cool project funded in part by NSF, DOE, LBNL, and the Bechtel Corporation)

• Online tutorial on using an SC001 chart

• Heavens Above (satellite, HST, ISS, iridium flare, etc. predictions for your area)
• Skyview
• Satellites
• AstroWeb
• Phil Plait's Bad Astronomy
• Excellent articles for beginners from Sky and Telescope
• Star-finding with a planisphere
• One free sky map each month
• USNO Data Services (Sun/Moon, Rise/Set, Twilight, Eclipses, Transits, other astronomical phenomena)
• Powers of 10 Tutorial
• Retrograde Motion Animated Demonstration
• Foucault Pendulum and the rotation of the Earth (from the California Academy of Sciences)
  • Diagramming the plane of oscillation on the floor
  • How position on the Earth affects the apparent change of the pendulum's plane of oscillation
• Visible spectra of the elements, absorption and emission
• Windows to the Universe
• Cosmic Evolution, from the Wright Center for Science Education at Tufts University
• Moon phase calculator
• the Hubble Deep Field
• SOHO (the Solar and Heliospheric Observatory)
• 3D Guide to the Galaxy by NASA's PlanetQuest
• excellent Moon map
• Astrophysics Links & Resources
• table of the 88 official constellations (note the mythology sections are somewhat information-light)
• Moon Phases Pictures (also has links to solar system sites)
• Fourmilab Switzerland has tons of cool resources not limited to astronomy
  • click "Astronomy and Space" from the "Science" menu at left to get those specific resources
• SpaceWeather.com
• NASA's Planetary Photojournal
• The Nine Eight Planets: A Multimedia Tour of the Solar System
• NASA - JPL Solar System Simulator
• The official U.S. Time
• track spacecraft at NASA
• NASA - JPL Space Calendar
• TerraServer - USA
• A View from the Back of the Envelope
• Runaway Universe from NOVA Online
• Night Sky Live (live webcams at night from observatories around the world)