Examples of powerful LaTeX packages and techniques in use — a great way to learn LaTeX by example. Search or browse below.

ellipsis -- pakietomat.wordpress.com
Pakiet modyfikuje polecenie \dots, ulepszając nieco jego działanie. Użycie pakietu ellipsis sprawia, że odstępy pomiędzy kropkami w wielokropku oraz odstęp samego wielokropka od otaczających go słów są lepiej ustalane.
Czytaj więcej na: http://pakietomat.wordpress.com/2014/06/07/ellipsis/
pakietomat.wordpress.com

multicol --- pakietomat.wordpress.com
Pakiet multicol pozwala na skład tekstu w wielu kolumnach. W ramach jednego dokumentu możemy fragmenty tekstu składać w różnych konfiguracjach składu wielokolumnowego (np. jeden fragment w jednej kolumnie a drugi w dwóch).
Czytaj więcej na: pakietomat.wordpress.com
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Typesetting scholarly critical editions with reledmac
reledmac is a package for typesetting scholarly critical editions, replacing the established ledmac and eledmac packages. The package supports indexing by page and by line numbers, and simple tabular- and array-style environments.
Maïeul Rouquette (uploaded by LianTze Lim)

Package example using the rubikcube package
Example - the `superflip' configuration
Once you have mastered Rubik's cube, then an interesting exercise is to generate
the so-called `superflip' configuration, in which all the corners are correctly
solved, while all the edges are flipped.
For the impatient, the superflip sequence of 24 quarter-turn rotations (listed on Randelshofer's website) is as follows. This converts the solved cube on the left into the configuration shown on the right.
Rubik package on CTAN: http://www.ctan.org/tex-archive/macros/latex/contrib/rubik
Apostolos Syropoulos, Dick Nickalls

forest -- pakietomat.wordpress.com
Pakiet forest pozwala na rysowanie prostych schematów o strukturze drzewa.
Czytaj więcej na: pakietomat.wordpress.com
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shadow -- pakietomat.wordpress.com
Pakiet dostarcza makropolecenia \shabox pozwalającego zrobić cieniowaną ramkę wokół tekstu.
Czytaj więcej na: http://pakietomat.wordpress.com/
pakietomat.wordpress.com

Simple example of alpha-numerical section levels in LaTeX
A minimal example showing how to choose different options for section and subsection labelling (including upper and lower case Roman numerals) by redefining the appropriate commands.
Source: http://www.latex-community.org/forum/viewtopic.php?f=19&t=17690
Stefan Kottwitz

Track Changes in LaTeX with trackchanges
Note: TrackChanges is not part of TeX Live, and hasn't been updated in a while. Instead, changes.sty, a package for the same purpose, is available in TeX Live and more recent. You can find the example template here.
TrackChanges is a package for collaboratively editing LaTeX documents which allows multiple editors to make changes & add annotations to a document.
Here we present a short example of it's use, which you can use as a template to get started.
writeLaTeX

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WriteLaTeX