I took this list from a special section of the Wall Street Journal titled "Technology," dated 11/17/97. I haven't yet followed all of these links myself; no guarantees or implied endorsements!
Yuckiest site on the Internet -- Attempts to satisfy children's curiosity about strange, yucky, or gross aspects of the world.
The Why Files -- The science behind the news, funded by the NSF
Cells Alive -- Information, with animated images, about cells of the immune system, bacteria, and parasites
Exploratorium -- The cutting-edge science museum in San Francisco, pioneers in interactive exhibits. This site has a long list of links to other instructional Web sites.
Windows to the Universe -- Introduction to the universe, including Earth and its neighbors, space missions, and astrophysical phenomena
Hubble Space Telescope -- Live from the far-seeing eye in the sky, with a chat area for teachers
The Stone Pages -- Tour of stone-age sites in Scotland, Ireland, and England
Volcano World -- Volcanic information
ThinkQuest -- Links to lots of exemplary educational projects on the web
American Treasures -- A library of congress exhibit culled from its collection, including Jefferson's handwritten draft of the Declaration of Indepence, Jelly Roll Morton's early compositions
Project Perseus -- Resources for the study of the ancient world: texts, translations, maps, illustrated art catalogs
World of Vikings -- Viking history and culture
Web Museum -- Digital versions of famous paintings
Vision and Art -- Tutorial on sensation and perception, with explanations of things like perspective
Shakespeare links -- Links to Shakespeare-related sites on the web