Fun sites and games
Basics of Space Flight
Astro-Venture - Search for and design a habitable planet
Planet Explorer

Projects
Galaxy Zoo
Stardust@Home
Imagine the Universe
NASA Quest

Exploring Careers
A New Universe to Explore: Careers in Astronomy
The Astronomy Cafe

Homework Help
Amazing Space Homework Help
The Astronomy Cafe

Tutorials for high school students or adults:
From Stargazers to Starships
The Exploration of the Earth's Magnetosphere
The Great Magnet, the Earth