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`

or`texdoc 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`

and`texdoc 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 today`texdoc`

understands commonly used aliases.