The Challenger Learning Center: The Challenger Learning Center hosts a variety of space-related science and engineering programs, particularly a simulated space mission complete with a Mission Control Center. It is one of approximately 50 in the world and is a part of the Challenger Center for Space Science Education.
The Challenger Center for Space Science Education: The parent organization of the Challenger Learning Center, it was founded in honor of the astronauts who lost their lives in the space shuttle Challenger accident in 1986. Their website has some very useful information for educators.
NASA Education Materials: NASA has some great classroom activities and lesson plan ideas.
HubbleSite: A whole website dedicated to the Hubble Space Telescope, complete with activities and lessons and an archive of the images taken by the telescope.
Mars Exploration Program: The site for NASA's Mars Exploration Program not only is a great resource for information on Mars or the missions to the red planet, but also for teaching materials.
The St. Louis Science Center: The SLSC has an entire wing dedicated to space exploration, as well as their own planetarium.