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.
A good place to also find the Mythology connection to the planets.

Solar System Exploration. From NASA, lots of facts on each planet.

National Geographic. Solar System.

Exploring the planets:  Our Solar System. This is a good source for discoverer information, as well as factual information.

Planetary Fact sheets. Lots of hard details, for those who are REAALLY into the hard science of Astronomy.

Probes and Fly-by Spacecraft. A short list from NASA of early planetary probes.

Planetary Science Spacecraft. A comprehensive list of planetary probes.

Planetary Missions (NASA). A very comprehensive list of past, present and future planetary missions. Contains detailed information.

What's in a Name on the Planets? The vocabulary of planetary exploration.

NASA Welcome to the planets. A good source for high quality images from satellite fly-bys.

Planetary Mythology:

Windows to the Universe: Mythology. Planetary mythology.