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

Poker Theorems
A simple trick to decorate Theorem-like environments with poker suits QED symbols.
I did not come up with this theorem decoration style (I've first seen it here) nor with the whole code (I salvaged it from TeX StackExchange and other sources over the years). This is just my current implementation of the code.
Níckolas Alves

Flag of the United States
Flag of the United States of America, according to official government specifications
Senan Sekhon

Espectro de OFDM 2
Esta es una rutina modificada del ejemplo publicado por Overleaf en este enlace https://es.overleaf.com/articles/espectro-de-ofdm/xxcsbsjjtbcb. Hay unas pocas diferencias relacionadas con el color de fondo y los bordes del gráfico, además del uso de la orden \addplot +[opciones] gnuplot [raw gnuplot] {}. El gráfico está basado en el que se muestra en la página 642 del texto "Digital Modulation Techniques, 2nd Edition" de la editorial Artech House, Inc. de Fuqin Xiong.
Ramón Jaramillo

Thai template example with polyglossia and xelatex
Thai template example with polyglossia and xelatex
LianTze Lim

Tracking changes in LaTeX with "changes" package (with author avatars)
The changes package, distributed with TeX Live, allows the user to manually markup changes of text, such as additions, deletions, or replacements. This example customizes the package to show author annotations with avatars. (Here's a more conventional example without the avatars. )
LianTze Lim

Tracking changes in LaTeX with "changes" package
The changes package, distributed with TeX Live, allows the user to manually markup changes of text, such as additions, deletions, or replacements. Changed text is shown in a different colour; deleted text is crossed out. The package allows definition of additional authors and their associated colour. It also allows you to define a markup for authors or annotations. (Here's another customised example using avatars for the author annotations.)
LianTze Lim

Example of using wiki markup in LaTeX documents
A simple example demonstrating how to use some wiki markup syntax in LaTeX, using wiki.sty from the nicetext bundle. Be careful — not everything works, and some commands may break! See http://mirrors.ctan.org/macros/latex/contrib/nicetext/doc/wikicheat.pdf for a cheat sheet. If you're looking for markdown, check out this example!
LianTze Lim

Psfrag Example
Use the psfrag package to replace strings in EPS images, so that they are typeset with the same body font as your LaTeX document. Note that your project needs to be compiled with the LaTeX dvipdf engine. To configure this, after creating a new project, click on the Overleaf menu icon above the file list panel, and make sure the "Compiler" setting is set to "LaTeX". (The sample image is taken from the psfrag package.)
LianTze Lim

Beautiful ornaments with pst-vectorian
Use PSTricks to draw ornaments (a substantial repertoire of ornaments is provided). See the package documentation for a list of all the ornaments.
LianTze Lim (examples from the package documentation)