Project Euler

Project Euler is a well-known website that is home to a multitude of math & computer programming problems. From the front page:

The intended audience include students for whom the basic curriculum is not feeding their hunger to learn, adults whose background was not primarily mathematics but had an interest in things mathematical, and professionals who want to keep their problem solving and mathematics on the edge.

I love solving these problems in my spare time. It’s even more challenging if you use a programming language that you’re not familiar with.