[joe_the_user]

Honestly, I would suggest looking at Wikipedia proper. It's coverage of mathematical topics seems to be currently quite good. You can generally easily google the proofs and the details for topics presented without these. The web altogether is goldmine of mathematical knowledge today imo.

Most things aren't explained "down to the elementary level" but anyone who spends significant time on this stuff will reach a higher level and appreciate not having the same elementary explanations repeated over and over.