The Nine Planets: A Multimedia Tour of the Solar System. Loads of information on each planet. This site has been updated in reference to Pluto no longer having the status of a planet.
http://nineplanets.org/
A good place to also find the Mythology connection to the planets.

Solar System Exploration. From NASA, lots of facts on each planet.
http://solarsystem.nasa.gov/index.cfm

Solar System Quick.
http://www.solarsystemquick.com/

Exploring the planets:  Our Solar System. This is a good source for discoverer information, as well as factual information.
https://airandspace.si.edu/exhibitions/exploring-the-planets/online/ss/

Planetary Fact sheets. Lots of hard details, for those who are REAALLY into the hard science of Astronomy.
http://nssdc.gsfc.nasa.gov/planetary/planetfact.html

Probes and Fly-by Spacecraft. A short list from NASA of early planetary probes.
https://airandspace.si.edu/exhibitions/exploring-the-planets/online/tools/tools_probes.html

Planetary Science Spacecraft. A comprehensive list of planetary probes.
http://nineplanets.org/spacecraft.html

Planetary Missions (NASA). A very comprehensive list of past, present and future planetary missions. Contains detailed information.http://nssdc.gsfc.nasa.gov/planetary/projects.html

What's in a Name on the Planets? The vocabulary of planetary exploration.
http://www.uwgb.edu/dutchs/planets/whatname.htm

NASA Welcome to the planets. A good source for high quality images from satellite fly-bys.
http://pds.jpl.nasa.gov/planets/

Planetary Mythology:

Windows to the Universe: Mythology. Planetary mythology.
http://www.windows.ucar.edu/tour/link=/mythology/mythology.html

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