For basic, useful information:
The Planetary Society
The NASA Homepage
Welcome to Your Astronomy Lessons: Helpful hints for observing as well as
basic astronomy information.
Welcome to the Planets: Information about the planets, and also about the
spacecraft that have explored them.
The Nine Planets
A Space Library: Space images using NASA space missions and computer
graphics. Interactive simulators.
Phil Plait's Bad Astronomy: The Home Page Good site for explanations of
astronomy myths and misinformation in movies, books, and the news, among
other places.
Jim Heath's Starry Site
For great pictures:
Astronomy Picture Calendar
STScI/HST Pictures: Hubble Space Telescope Public Pictures.
Cyber-Cards.com: Space shots.
For fun things to do:
Introduction to Windows to the Universe
SETI@home: Search for Extraterrestrial Intelligence at Home: How to set up
your computer to analyze SETI data.
Absolute Beginners Astronomy and Telescopes Page:
This one is for those
interested in learning how to become an observer or an amateur astronomer.