My code snippets, inspired by Chris Albon's very useful site.

Python

Open (and close) a file with the with statement.
List comprehension with a conditional statement.
List files in a directory.
Force reload of a module.

Pandas

Create Categorical Dtypes in Pandas

Matplotlib

Add text to a plot with relative position.
Cool way to make plots that look hand-made.
Modifying tick labels.

Bokeh

Put bokeh plot in Jupyter Notebook.

Scikit Learn

Regular Expressions

Javascript

D3

enter() and exit() explained.
D3 Color Schemes
Get unique values from a Javascript object
Rotate axis ticks marks
Make months on axis 3 letters only.
Rotating text in D3