Constructing an SF Universe
Excellent article by professional, published science fiction author, S. Andrew Swann.

Pegasus Nest: Worldbuilding
The page is oriented toward fantasy/science fiction writers, BUT the principles of good worldbuilding should apply to rpg's as well.

Planetary Fact Sheets
Extensive physical data describing the planets, satellites, and comets of our solar system, courtesy of NASA's National Space Science Data Center. Potentially useful to designers of science fiction settings and the detail-minded.