Astronomy
Astronomy Online
For those new to observational astronomy.
Astroweb:
Astronomical Internet Resources
This highly reputable site is searchable by keyword, by topic list, or
by its index
Center for Earth and Planetary
Studies
This site is part of the Collections and Research Department
of the National Air and Space Museum, Smithsonian Institution. Extensive
image collection. Searchable.
Digital
Librarian: a Librarian's Choice of the best of the Web: Astronomy
A large annotated collection of links on astronomy and aerospace.
Hubble Space Telescope
NASA's Space Telescope Science Institute site. Searchable.
Jet Propulsion Laboratory
Site maintained by NASA and Cal Tech with info on JPL, solar system,
and related technology.
NASA Homepage
NASA updates on space exploration and related sciences.
Physics
and Astronomy Reference 
A searchable site with many resources that are hyperlinked to provide
additional information.
Royal Observatory Greenwich
Sponsored by the National Maritime Museum, this site includes downloadable
leaflets, night sky links, online exhibits, and special educational programs.
Searchable.
Sky
and Telescope
This is a portal to online astronomical information. It includes
links to other relevant sites, astronomy news, an almanac, even instructions
on using telescopes.
Sloan Digital Sky
Survey
The Sky Survey provides great graphics! The site also includes basic
and advanced astonomy projects..
Wikisky.org
Interactive, free, noncommercial Web site that aims to consolidate
astronomical information.