Created by the former CEO of Autodesk (AutoCAD Co.), also has other tools and downloadable applications.

Operated by JPL, generates ephemerides for any solar system object with all the options you can think of.

The JPL program that searches for asteroids/comets and assesses collision risk. Site has tools for viewing trajectories and generating ephemerides.

FITS is a common file format for images taken with telescopes. This is a free graphical viewer and editor by NASA.

Well known for real-time info/charts of where and when to look at ISS and other satellite orbital passes.

The free online telescope located in the Canary Islands. Very cool, but it can take weeks to get the image from a job taken.

Free software for stacking multiple astronomy images to improve features, especially in planets. (Windows only)

Open-source Mac OS X software for stacking multiple planetary astronomy images to improve features. (Have not tried this myself)

A different astronomy and space science related image is featured each day, along with a brief explanation.