Cfrgtkky

I almost always load microtype. It plays with ever-so-slightly shrinking and stretching of the fonts and with the extent to which text protrudes into the margins in a way that yields results that look better, that have fewer instances of hyphenation, and fewer overfull hboxes. It doesn't work with latex, you have to use pdflatex instead. It also works with lualatex and (protrusion only) with xelatex.

The family of AMS math packages. At least amsmath and amssymb. Also amsthm if I need theorems and the class I'm using doesn't already define them.

Particularly for writing equations, the AMS packages define a rich set of environments to group and align formulas in many different and useful ways. I also like that it encourages the use of semantic commands (e.g. the cases environment) over syntactic commands (e.g. a left{ followed by an array).

Its documentation can be found running texdoc amsldoc on a command line.

I use hyperref for setting PDF metadata and to create links, both within the document and for clickable URLs. Even Elsevier has used urlbst to update their bibliography style to support URLs and DOIs; hyperref does the actual work of rendering url = and doi = BibTeX fields into clickable PDF links.

For citations and bibliographies, biblatex is the package of my choice. Key points:

• biblatex includes a wide variety of built-in citation/bibliography styles (numeric, alphabetic, author-year, author-title, verbose [full in-text-citations], with numerous variants for each one). A number of custom styles have been published.




• Modifications of the built-in or custom styles can be accomplished using LaTeX macros instead of having to resort to the BibTeX programming language.




• biblatex offers well-nigh every feature of other bibliography-related LaTeX packages (e.g. multiple/subdivided bibliographies, sorted/compressed citations, entry sets, ibidem functionality, back references). If a feature is not included, chances are high it is on the package authors' to-do list.




• The babel package is supported, and biblatex comes with localization files for about a dozen languages (with the list still growing).




• Although the current version of biblatex (2.8a) still allows to use BibTeX as a database backend, by default it cooperates with Biber which supports bibliographies using Unicode. Biber (currently at version 1.8) is included in TeX Live and MiKTeX. Many features introduced since biblatex 1.1 (e.g., advanced name disambiguation, smart crossref data inheritance, configurable sorting schemes, dynamic datasource modification) are "Biber only".

The todonotes package is a must have in all my documents.

usepackage{todonotes}

The package enables you to insert small notes in the text marking things to do in the document. Something like

todo{Rewrite this answer ldots}

At any location in the document a list of the inserted notes can be generated with the

listoftodos 

command.

One package that’s really general purpose is nag: It doesn’t do anything, per se, it just warns when you accidentally use deprecated LaTeX constructs from l2tabu (English / French / German / Italian / Spanish documentation).

From the documentation:

Old habits die hard. All the same, there are commands, classes and packages which are outdated and superseded. nag provides routines to warn the user about the use of those. As an example, we provide an extension that detects many of the “sins” described in l2tabu.

Therefore, I now always have the following in my header (before the documentclass, thanks qbi):

RequirePackage[l2tabu, orthodox]{nag}

It’s a bit like having use strict; in Perl: a useful best practice.

I'm surprised that no one has mentioned

usepackage[margin=1in]{geometry} % set page margins automatically 

This is in every document I write (with varying margins, of course.)

Another essential package combination is

usepackage{booktabs}

usepackage{array}

The booktabs package creates much nicer looking tables than the standard latex tables; the array package's ability to create custom columns is invaluable for formatting tabular material on a per-column basis.

I nearly always use the tikz package. Once you learn how to draw with it, you can do almost any vector graphic you need.

Since my files nowadays has UTF-8 character encoding, I use this

usepackage[utf8]{inputenc}

usepackage{siunitx}

siunitx, for typesetting units and especially for the "S" column type, which allows numbers in tables to be easily aligned, e.g. on the decimal marker.

The 'rich' document classes such as memoir and KOMA-Script include a lot of functionality that is not available from the LaTeX kernel. So the packages you load when using the article class might be rather different from those when using memoir. A lot of packages that get used by many people with the base classes (things like float, caption, tocbibind and titlesec) are covered by the richer document classes.

usepackage{graphicx}

For including figures, rotating or scaling text. I also use the graphicspath command to specify a subfolder to help organize my figures and so I can easily change between, for example, a set of figures for internal used (with extra info) and final versions for distribution.

usepackage{lmodern}  % better i18n Postscript version of Knuth's cm fonts

In addition to many packages already listed here, I always include mathtools. It provides implementations of mathclap (and similar commands) as well as nice extensible arrow.

I can't live without listings --- pretty-printing (colours, formatting and all) algorithms and code is indispensable --- in pretty much any programming languages and dialects under the sun. Plus, I can import a source file directly from the repository, and the latest version will be automatically rendered.

The package xspace lets you define commands that don't eat up whitespace after them. So you can define an abbreviation like

newcommand{sA}{mathcal{A}xspace}

and then you can type objects of sA are called widgets instead of objects of sA are called widgets.

First line of the document should be

RequirePackage{fixltx2e}

documentclass{...}

, which fixes a few things in the LaTeX2e kernel.

Due to LaTeX's stability policy, these corrections have not been incorporated into the LaTeX2e kernel, but this package does things most people would agree are bugfixes. So to load this package is always recommended for newly created documents. The corrections have no commonalities, but the package's description has a nice summary:

• ensure one-column floats don't get ahead of two-column floats;


• correct page headers in twocolumn documents;


• stop spaces disappearing in moving arguments;


• allowing fnsymbol to use text symbols;


• allow the first word after a float to hyphenate;


• 
  emph can produce caps/small caps text;


• bugs in setlength and flushbottom.

EDIT 27.01.2016:

This package is obsolete for LaTeX releases after 2015. See latexrelease.pdf.

For papers on the arXiv (maths, physics and computer science mostly) there's a list of packages sorted by frequency of use.

The top twenty packages are:

1. article


2. graphicx


3. amssymb


4. amsmath


5. revtex


6. revtex4


7. epsfig


8. amsfonts


9. bm


10. latexsym


11. amsart


12. dcolumn


13. amsthm


14. graphics


15. aastex


16. amscd


17. epsf


18. color


19. aa


20. times

I use url to typeset urls.

For quickly setting multicolumn text in a single column document, the multicol package is another package that I use all the time.

usepackage{multicol}

To use the palatino font (it's just a nice looking font)

usepackage[sc]{mathpazo}

Note that the old palatino package is deprecated.

usepackage[parfill]{parskip} 

I much prefer no indentation and space between paragraphs, so the parskip package is a must for me!

I almost always find myself using a tabularx environment as opposed to the regular tabular environment, as it allows for greater dynamism in column widths.

Nothing surprising here: I use natbib, hyperref and hypernat together.

Natbib for referencing.

Hyperref adds bookmarks for sections and lists and turns references and urls into links.

Hypernat allows natbib and hyperref to work together. -- Note (added 2015/02/11): natbib and hyperref have been working together just fine for at least ten years. hypernat is no longer needed for any TeX distribution with a vintage more recent than ca 2002.

I almost always use the enumitem package, which makes it much easier to make modifications to lists (especially enumerate lists). Most notably, changing the labels to something like (i), (ii), (iii) [no period] with this package is as easy as

begin{enumerate}[label=(roman*)]

    item The first item

    item The second item

end{enumerate}

Furthermore, the code above will automatically get nesting right. Before I started using this package, my preamble always included the awkward macro (necessary to change the references and eliminate the extra period in the list itself)

newcommand{setenumroman}{%

    renewcommand{theenumi}{(roman{enumi})}%

    renewcommand{labelenumi}{theenumi}%

}

which would break if I ever used it for a nested list (all the enumis would have to be changed to enumiis, if I understand correctly).

The enumitem package is quite flexible; another option I sometimes use is [wide], which makes a list look like part of the body of the text (with numbers/labels at the beginning of relevant paragraphs).

To make sure you have ISO formated dates (YYYY-MM-DD).

usepackage[english]{isodate}

or

usepackage{datetime}

renewcommand{dateseparator}{-}

newcommand{todayiso}{theyear dateseparator twodigitmonth dateseparator twodigitday}

Another package I use is float. It allows for the placement H for floats, which is somewhat equivalent to h!, but a bit stronger, making sure the figure or table goes exactly where I want it to be.

For mathematical texts I instead use amsmath & Co. One very useful package is onlyamsmath. I load it as

usepackage[all,warning]{onlyamsmath}

So it looks for $$..$$, eqnarray and produces a warning if some of them are used. If you left out warning, it will result in an error and compile will stop. This package is normally very useful if you edit a text with many authors.

Edited by doncherry: Removed packages mentioned in separate answers.

The complete header Part of my header for most of my documents looks as follows:

documentclass[ngerman,draft,parskip=half*,twoside]{scrreprt}

usepackage{ifthen}

For some things I need if-then-constructs. This package provides an easy way to realise it.

usepackage{index}

For generating an index.

usepackage{xcolor}

xcolor is needed by several packages. For some historical reason I load it manually.

usepackage{babel}

usepackage{nicefrac}

nicefrac allows typesetting fractions like 1/2. It is sometimes more readable than frac.

usepackage[T1]{fontenc}

usepackage[intlimits,leqno]{amsmath}

usepackage[all,warning]{onlyamsmath}

This package warns if non-amsmath-environments are used.

usepackage{amssymb}

usepackage{fixmath}

Provides ISO conform greek letters.

usepackage[euro]{isonums}

Defines comma as decimal delimiter.

usepackage[amsmath,thmmarks,hyperref]{ntheorem}

for Theorems, definitions and stuff.

usepackage{paralist}

Improves enumerate and itemize. Also provides some compact environments.

usepackage{svn}

I work with VCS and svn displays some informations (keywords) from SVN.

usepackage{ellipsis}

corrects dots

DeclarePairedDelimiter{abs}{lvert}{rvert}

DeclarePairedDelimiter{norm}{lVert}{rVert}

These are the definitions for absolute value and norm.

SVN $LastChangedRevision$

SVN $LastChangedDate$