Astrometry Department of the U.S. Naval Observatory (USNO).
Astrometry is the branch of astronomy that deals with the measurement of the positions and motions of the celestial bodies. This site includes many features, projects, catalogs, and photos from the instrument shop.

Gazetteer of Planetary Nomenclature.
From the U.S. Geological Survey (USGS), this gazetteer contains detailed information about all names of topographic and albedo features on planets and satellites (and some planetary ring and ring-gap systems) that the International Astronomical Union (IAU) has named and approved from its founding in 1919 through the present time.

Hubble - a View to the Edge of Space.
Sponsored by the National Science Foundation, this site dissects Hubble's images and explains what they tell us about the universe.

Hubble Deep Field.
The Hubble telescope's deepest-ever view of the universe unveils myriad galaxies. Be patient. These pages take time to load, but are worth the wait. Formats are JPEG, GIF, and TIFF.

Night Sky Network.
A partnership of amateur astronomy clubs, NASA, Astronomical Society of the Pacific, and the Astronomical League. The Night Sky Network provides training and materials on themed topics, a means to network with other amateurs, and effective ways to communicate with varied audiences.

Open Yale Courses - ASTR 160: Frontiers and Controversies in Astrophysics.
Free online course from Yale University, including video lectures, audio and text files. No registration is required but, no credit is given. This course focuses on Extra-Solar Planets, Black Holes, and Dark Energy. Particular attention is paid to current projects that promise to improve our understanding significantly over the next few years.

Views of the Solar System.
Contains the latest scientific information on the Sun, planets, moons, comets, asteroids, and more. Includes the history of space exploration, rocketry, early astronauts, space missions, and spacecraft. Many subjects include multimedia files.

Your Sky.
An interactive planetarium where you can produce maps for any time and date, viewpoint, and observing location. If you enter the orbital elements of an asteroid or comet, Your Sky will compute its current position and plot it on the map. Each map is accompanied by an ephemeris for the Sun, Moon, planets, and any tracked asteroid or comet.