Atomic Rockets
projectrho.com/public_html/rocket
A huge reference on realistic spaceship design, space travel physics and settings for science fiction writers and game designers, part of Project Rho.
Why it is useful
A deep resource for anyone who wants the science to be right, with pages on propulsion, weapons, orbits and space habitats. It is full of sharp, blunt commentary that is as entertaining as it is informative.