GLOBE at Night is a great GIS site with education in mind. Various sponsors like NASA and ESRI have helped bring this site to the web. It has a fully functional database that allows for layers and zooms. Overall, a neat site.
Here's a quick summary of what GLOBE is:
- GLOBE (Global Learning and Observations to Benefit the Environment) is a worldwide hands-on, primary and secondary school-based education and science program.
Also, check out NASA's VISIBLE EARTH site for a library of satellite images from various Space missions.