Today I had some Hungarian mathematical logic notes to type in Latex, and again I had to dig up the various references to get the proper symbols I needed.
I used the following pages:
amsmath is unique in that for example it explains how to use \begin{cases}
this one is a collection of lot of symbols, I found \mathcal here
An article from Wikipedia about logic symbols
The Math chapter of the well-known LaTeX wikibook
A set of macros making easy to declare lemmas, definitions, proofs.