The most widely used quick reference among plasma physicists is the NRL Plasma formulary. It is available on the web or can ordered as a convenient pocket-sized booklet.
My start at A Physics Formulary to supplement the NRL Plasma Formulary is also available in a foldable booklet version.
Johan Wever has a more
comprehensive
physics
formulary,
math
formulary, and
general physics related
links.
Review
of Particle Physics, includes physical constants, tables, and reviews
of cosmology, probability, and statistics, etc.
Eric Weisstein's / Wolfram Research's World of Physics,
MathWorld,
Scienceworld.
Planet Math (interesting wiki using
LaTeX.)
Other references and links:
Concise physics summaries
John Baez's General
Relativity Tutorial.
While these references are useful, remember: don't believe everything
you read.
Jonathan Birge's formulas.
Wikipedia
Enclopedia Britannica
MIT Online
Courseware.
Connexions: Online Courseware (started by
Rice University)