Moon phase demonstration. A web site put up by NASA's Jet Propulsion Laboratory which offers a nice demonstration of moon phases, suitable for the classroom.
JPL's Education Gateway. A web site put up by NASA's Jet Propulsion Laboratory with a variety of educator information, including lessons plans for different grade levels.
Amazing Space.
A school-activity based site provide by the Space Telescope Science Institute.
Space Weather Center. A site that tells about space weather - how the Sun can affect us and what research is being done to become more aware of those impacts.
P.O.E.T.R.Y. A web site offered by NASA's Goddard Spaceflight Center providing information on auroral science and the Earth's magnetic field. Specific to grades 4-12.