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.
Document Layout
- The Memoir manual: Covers, in exquisite detail, how to produce just about any document using the
memoir
LaTeX class. (PDF 4.5 MB)
Mathematical Typesetting
- Math Mode: A document by Herbert Voß that explains how to typeset just about any mathematical expression in LaTeX. (PDF 2 MB)
Graphics
- The PGF Manual: Explains, with examples, how to draw just about any figure using PGF/TikZ. Also a good example of what awesome documentation should look like. (PDF 9.5 MB)