I've been helping out with some research for a school project on space and the environment the last few days. Although I have often explored their many web sites, this project really highlighted for me the amazing amount and high quality of the on-line resources available from NASA. Whether it's historical information on the Apollo program, educational support, current mission information, or regularly updated Earth sciences data, NASA has it all.
The only negative thing I would say is that because the various NASA centers have their own web sites, it can sometimes be hard to find exactly what you need. But Google usually comes to the rescue here, since a search with the keyword site:nasa.gov is usually more effective than NASA's own site search feature in finding the "good stuff." For example, searching for explorer 1 site:nasa.gov returns top-10 results from jpl.nasa.gov, gsfc.nasa.gov, history.nasa.gov, and solarsystem.nasa.gov.
One area I had not explored much before is NASA's various Earth science sites. One particularly interesting one is on the water and energy cycle. It includes a fascinating (but big - 45 MB) animation of the major components of the global water cycle.