Planet rendering:

Sean O'Neil gamasutra article (Needs login)
Dave Hill research project
Bad Dog Real-Time Procedural Planet
Jon Harrop Real-time Level of Detail Planet Rendering

Planet heightmaps and textures:

Space Graphics, textures of Mars, Earth and asteroids.
Planetary image maps features planet textures and heightmaps.
Maps of the solar system, textures of solar system planets.
Blue Marble, high-res textures of Earth.
OpenUniverse maps, high-res textures of the Moon and Mars.

Planetary data:

Nirgal.net, extensive collection of information about Mars. In French.
Planets: Physical Data, JPL/SSD data.

Software:

Orbiter, space flight simulator. Orbiter enables you to pilot spaceships (real or sci-fi). It is a kind of "hard-SF" program relying upon accurate physics.
Open Universe, a solar system simulator. It implements the proper motion of planetary bodies and some asteroids. You can freely travel around to view the bodies.
Celestia, space simulation. You can not only view the solar system but also others stars (108,000 stars referenced, some with extrasolar planets). Eclipses are rendered.

Misc:

Vers les étoiles, site du newsgroup français fr.sci.astronautique.