NASA has a nice Solar System Exploration website
ASPIRE - Astrophysics interactive labs for teachers
http://aspire.cosmic-ray.org/
Spacelink: instructional materials from NASA
http://spacelink.nasa.gov/Instructional.Materials/
NASA Spitzer Space Telescope
http://www.spitzer.caltech.edu/
Multiverse is "A center of multicultural science education to support diversity in earth & space science."
Ancient Observatories
https://www.exploratorium.edu/ancientobs/