If you're here, you should probably go here.

Matrix Reference Manual, by Mike Brookes
Wolfram Alpha
Mathworld, by Eric Weisstein
Non-Uniform Random Variate Generation Textbook, by Luc Devroye
On-line Statistics Textbook, by Statsoft
Public Library of Science

Life, Social, Cognitive and Technology Sciences

I don't know where else to keep the list of books that I thought were worth reading, so I'll just maintain it here. (I really appreciate receiving recommendations for other good work on these topics.)

Cognition and the brain

Social science, history, technology

Evolution and life

Someday I'll figure out where I'm going with all of this ...