Links

NASA
Astronomy Picture of the Day: Learn astronomy while admiring the beauty and majesty of our universe.
Astronomy Magazine: An excellent magazine about astronomy.
Harvard-Smithsonian Center for Astrophysics
NSSDC - Lunar and Planetary Science
The ~~Nine~~ Eight Planets: All about our local corner of the universe, the solar system.
Stars and Constellations: A comprehensive resource, including information about most bright stars and all 88 constellations.
The Constellations and Their Stars: Even more information.
Messier Objects and NGC Objects: The most interesting deep-sky objects.
The Extrasolar Planets Encyclopaedia: A complete list of every extrasolar planet ever discovered.
Extrasolar Visions: Lots of great info and pictures (artistic renderings, but scientifically guided).
The Kepler Mission: A search for habitable, Earth-like planets.
The Hubble Space Telescope
The Spitzer Space Telescope
The Chandra X-Ray Observatory
Meteorites: What they are and how they're classified.
Scale of the Universe Videos: The Earth is a very small stage in a vast cosmic arena. Think of the rivers of blood spilled by all those generals and emperors so that, in glory and triumph, they could become the momentary masters of a fraction of a dot. Think of the endless cruelties visited by the inhabitants of one corner of this pixel on the scarcely distinguishable inhabitants of some other corner, how frequent their misunderstandings, how eager they are to kill one another, how fervent their hatreds. Our posturings, our imagined self-importance, the delusion that we have some privileged position in the Universe, are challenged by this point of pale light. Our planet is a lonely speck in the great enveloping cosmic dark. In our obscurity, in all this vastness, there is no hint that help will come from elsewhere to save us from ourselves. The Earth is the only world known so far to harbor life. There is nowhere else, at least in the near future, to which our species could migrate. Visit, yes. Settle, not yet. Like it or not, for the moment the Earth is where we make our stand.
--Carl Sagan; Virgo Cluster; Hubble UDF zoom-in; SDSS map galaxies; The Grand Scale of the Universe; The Known Universe

Earth Science Links

U.S. Geological Survey
Glaciology terms: Learn about moraines, eskers, etc. The Science Bowl loves these.
Palaeos: The history of life on Earth.
Mineral Gallery: Learn about different minerals and how they're classified.
Rock Gallery: Learn about different rocks and how they're classified.
The Cloud Atlas: Learn about clouds and how they're classified.
More about clouds
Online Textbook

Biology Links

The Tree of Life: Provides information about biodiversity, the characteristics of different groups of organisms, and their evolutionary history.
Animal Diversity Web: An online database of animal natural history, distribution, and classification.
ZooBank: The online registry for zoological nomenclature.
All About Birds: An online guide to birds and bird watching.
FishBase: More than you ever wanted to know about the fishes of the world.
The Plant List: All known plant species.
The Plant Kingdom
Fungi and more Fungi
Online Textbook

Math

Siemens Westinghouse Competition

American Mathematics Competitions

The Mathematical Association of America.

AMC Practice

Practice Problems

Competition Links

CAML (California Mathematics League)
Mandelbrot Competition
Stanford Math Tournament
Tokutomi Mathematics Contest
Bay Area Mathematical Olympiad (BAMO)

MathWorld

"The world's most extensive mathematics resource."

Art of Problem Solving

MathLinks

Computer Science

USA Computing Olympiad (USACO)
American Computer Science League (ACSL)

Random Interesting Things

3D Mandelbrot Fractals: Intriguing.
How does this happen?: Volcanic lightning.

Other

Wikipedia: The most reliable source on the web.^{[citation needed]}; But seriously, many of their science articles are actually pretty good.
Science Jokes: Someone once said that the point of an education was so that you could understand more jokes.
xkcd: Why not?