Data
Links to many public economic data sources, via Amherst College Department of Economics (no need to redo the list here)
Quandl - search over 5,000,000 datasets, also a Stata package to access Quandl datasets
A really cool set of data science projects, by Noah Veltman
A large variety of "toy" datasets to practice modeling.
A really nice set of datasets to explore/practice, by Melanie Walsh.
Python
Python for Economists
Stats in Python
Quantitative Economics (Sargent and Stachurski)
Datacamp
PEP 8 - Style Guide for Python Code
Matplotlib Colors
python4econ blog
R
An excellent primer on R written by Don Edwards
The R Project for Statistical Computing
One Page R: A Survival Guide to Data Science with R
Tidyverse: dplyr
Analyze survey data for free - R scripts
Math/Puzzles
An excellent resource on math history
Lateral thinking puzzles
Interactive math museum
Nim game
Trust game