The Calendar Sky - Customize your own astronomical and space calendar with predictions for your geographical location, with astronomical events (eclipses, planetary conjunctions, etc.), International Space Station passage, and much more.
Astronomy textbook from open source Wikibooks - free books which can edited (added to or corrected) by other visitors to the Wikibooks website
Spaceweather PHONE monthly service that will call you when any new space science or astronomy events (such as appearance of a new bright comet) occur.
AAVSO - American Association of Variable Star Observers - Information for observing and measuring the brightness curves of variable stars.
Association of Lunar and Planetary Observers - Association of Lunar and Planetary Observers is one of the oldest amateur astronomy organizations; learn all about viewing and doing serious scientific research on solar system objects (planets, the Sun, asteroids, comets, etc.)
Astronomical League - Confederation of U.S. astronomical societies inviting you to browse their information on astronomy and astronomical societies.
American Meteor Society - The American Meteor Society is a collaborative effort between amateur and professional astronomers, for the purpose of conducting visual observations of both meteor showers and meteors appearing at random (sporadic meteors).
International Dark Sky Association - A non-profit member organization that teaches others how to preserve the night sky through factsheets, law references, pictures, and web resources.