NASA has a Space Education Web site for educators to use.
The resources are sorted by type and grade level.
There are videos, lesson plans, resource links, news and interactive sites on NASA programs and projects. There is also a Student site that students can explore.
I've used many resources from the site and find them well thought out and easy to use.
The one I have used the most is the Rocketry resources. I do a final project at the end of the year in my Physics classes where they research rockets and the physics of rockets and then build and launch their own model rockets. The students can just go through the Rocketry site and learn about all the different aspects of rocekts. Its a fun, interesting way to end the year.