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)
Best Answer
For now, I will post reference manuals that haven't been mentioned here yet.
LaTeX
texdoc source2e
and enter a keyword to the search feature of the pdf browser.Presentations
texdoc beamer
.Document Layout
texdoc scrguien
ortexdoc scrguide
.Mathematical Typesetting
texdoc amsldoc
Symbols
texdoc symbols-a4
Hypertext and PDF features
The probably most important reference is the hyperref manual. It deals with nearly all you need to know about hyperlinks and PDF bookmarks. What it doesn't tell yet, may be read in the README file to that package.
texdoc hyperref/manual
andtexdoc hyperref/readme
Further there's the pdfTeX user manual.
texdoc pdftex-a
TeX
Bibliographies
texdc btxdoc
texdoc
is the tool for quick access, but one should know for what to look out. Earlier it was difficult because one had to remember complicated file names, but todaytexdoc
understands commonly used aliases.