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

Elements for LaTeX
Just a collection of the code I've found and use regularly in projects. Includes crazy shapes, graph paper, margin changes and alignment of equations.
Bon Crowder

Cardioid (from an idea of O. Reboux)
LaTeX is great for plotting mathematical shapes -- here we present an example of a cardioid.
A cardioid is a plane curve created by tracing a point on the perimeter of a circle rolling around a fixed circle of the same radius. Read more
Original source: http://www.altermundus.fr/pages/euclide.html
altermundus

Songs, Chords and Lyrics in LaTeX with the Songs Package
The songs package lets you create beautiful songbooks in LaTeX. You can typeset lyrics, lyrics with chords, and overhead slides for singing, all from a single master document. You can include both chord names and guitar tablature diagrams (but not full sheet music).
This example is based on an excerpt from the example songbook in the documentation for the songs package.
The Songs Package Project

Phép nghịch đảo
Cho ba điểm A, B, C thẳng hàng theo thứ tự đó và ba nửa đường tròn đường kính AB, AC, BC nằm về một phía của đường thẳng AB. Dựng đường tròn tiếp xúc với cả ba nửa đường tròn trên.
Nguyen Thai Son

Lá cờ Việt Nam
Code vẽ lá cờ Việt Nam dành cho SV ngành Toán ĐHSP để dạy lại cho học sinh cách vẽ lá cờ này.
Nguyen Thai Son

Culmus Fonts for Hebrew in LaTeX
This example shows how to use Culmus fonts to write Hebrew in LaTeX. The Culmus Project provides a collection of Hebrew fonts for GNU/Linux and LaTeX communities.
Overleaf (formerly writeLaTeX) provides Culmus Fonts 0.7-r1 (as of 26 Jan 2014), in addition to the standard LaTeX fonts for Hebrew. (If you remove \usepackage{culmus}, you can see the standard fonts instead.) Also note that your Hebrew text must be UTF-8 encoded, and that Overleaf also supports entry of right-to-left (bidirectional) text in your LaTeX source.
Overleaf (writeLaTeX)

graphql-higlight
Highlight your GraphQL code right in your LaTeX code. (See also this help article and this minted example.
NyanKiyoshi

Sample Rtex document
Very basic example of including R code in a LaTeX document.
Jeremy Foote

UQAC Exemple de base 2019
L'exemple de fichier pour l'atelier Latex 2019 à l'UQAC.
Émile Bélanger