Sun Course An almost complete course on the Sun, including both conceptual and mathematical presentations of a variety of topics.
The NRL Plasma Formulary -- A
wonderfully reference containing equations, formulas, constants, and
parameters from all branches of physics, though concentrating on
plasmas.
Physics of the aurora --
fancy interactive materials about the magnetosphere and aurora from
UCAR (local copy).
SPACE.com -- Commerical site with information about space