Grolier Online  - school use
http://go.grolier.com - use this address for home use
Includes 3 general encyclopedias, plus New Book of Popular Science.

The Nine  8 Planets
http://www.nineplanets.org/

Exploring the Planets: Our Solar System (Smithsonian National Air and Space Museum)
http://www.nasm.si.edu/ETP/ss/

KidsAstronomy.com
http://www.kidsastronomy.com/solar_system.htm

The Planets (Enchanted Learning)
http:www.enchantedlearning.com/subjects/astronomy/planets/

The Solar System (Astronomy Magazine)
http://www.astronomy.com/asy/default.aspx?c=ss&id=192

Solar System Exploration (NASA)
http://solarsystem.nasa.gov/planets/index.cfm

StarChild (NASA)
http://starchild.gsfc.nasa.gov/docs/StarChild/StarChild.html
Select Solar System, Level 1 or Level 2

Our Solar System (Windows to the Universe) http://www.windows.ucar.edu/tour/link=/our_solar_system/solar_system.html