Don't the name fool you, the USGS National Map Viewer is more than just a place to look at a map. The USGS National Map Viewer lets you choose from a huge library of datasets to display on a map. You can view the source information for each dataset. Additionally, you can choose the base map on which the datasets are displayed. If that's not enough to get you to try the USGS National Map Viewer, I should also tell you that you can draw on the maps, measure on the maps, and print your customized map displays. Watch this short video to get an idea of what is possible with the USGS National Map Viewer.
Applications for Education
The USGS National Map Viewer could be a great tool for students to use to make visual connections between the information provided in a dataset and the locations referenced in those datasets. For example, in the video above I applied the earthquake faults dataset to the map so that students can see where there is more or less seismic activity in the United States. On a related note, here's a nearly realtime USGS map of the latest seismic activity around the world.