This is a community wiki since there is no "one true answer"- if you find an answer that contains a lot of your choices, but is missing something you feel is incredibly useful. Feel free to add it. A short description of what topics the reference covers would be nice as well.
Organize by subject please.
For example: I always keep the following PDFs at my fingertips.
- The Memoir manual: Covers, in exquisite detail, how to produce just about any document using the
memoirLaTeX class. (PDF 4.5 MB)
- Math Mode: A document by Herbert Voß that explains how to typeset just about any mathematical expression in LaTeX. (PDF 2 MB)