General Tools

Center of mass calculator
Kepler's 3rd law (Java)
OJTA
val-vlabsdev

http://simbad.harvard.edu/sim-fid.pl
http://antwrp.gsfc.nasa.gov/apod/astropix.html (APOD)
http://www.fourmilab.ch/cgi-bin/Solar (Solar System Live)

Emission and False-Color

http://antwrp.gsfc.nasa.gov/apod/ap061020.html (Gum Nebula in false color)

http://antwrp.gsfc.nasa.gov/apod/ap051223.html (H-alpha in Rosette Nebula)

http://antwrp.gsfc.nasa.gov/apod/ap060324.html (Rosette in false color)

http://hubblesite.org/gallery/behind_the_pictures/meaning_of_color/eagle.shtml (Example of Hubble false coloring)

http://hubblesite.org/gallery/behind_the_pictures/meaning_of_color/hubble.shtml (Interactive: NGC 1512 through 7 filters)

http://hubblesite.org/gallery/behind_the_pictures/meaning_of_color/filters.shtml (Summary: NGC 1512 through 7 filters)

Satellites

http://science.nasa.gov/Realtime/JTrack/Spacecraft.html (satellite tracks)
http://science.nasa.gov/realtime/jtrack/3d/JTrack3D.html (3D satellite tracks)
http://www.fourmilab.ch/earthview/satellite.html (simulated view from satellites)

Solar System

Relative sizes solar system
Solar System Animator
Inner Solar System (1994-1998)

Retention of Atmosphere Animation