Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields.
MathOverflow is a question and answer site for professional mathematicians.
Prerequisites for the Langlands Program (by Anthony Knapp)
Steve Gelbart's Lectures on the Arthur-Selberg Trace Formula
J.S. Milne's Home Page (for various textbooks)
Robert B. Ash's Home Page (for various textbooks)
An elementary proof that zeta(2)=pi^2/6, in fact there is a list of such proofs here and here
Hyperbolic Functions:
Hardcore History Podcast (by Dan Carlin)
The History of Rome Podcast (by Mike Duncan)
The Revolutions Podcast (by Mike Duncan)
Our Fake History Podcast Historical Myths Relished and Ruined! (by Sebastian Major, which I believe to be a pseudonym)