From time to time I enjoy testing my skills on various programming puzzle sites, here’s a list of sites in no particular order.
Project Euler is a collection of math based coding challenges, where you can submit and verify your result, when you’ve found a solution.
I’m still a rookie on the site, but I have solved quite a few problems. Here’s my badge and a challenge to beat my score!
Advent of Code
This yearly tradition is very fun, and not quite as math heavy as Project Euler tends to be. A lot of the puzzles can be solved by traditional means, with familiar algorithms and datastructures.
Another site that focuses on honing your skills is http://codewars.com. It’s a novel puzzle site, with a martial arts theme, so leveling up means advancing your Kyū through the various colours and gaining honor. It’s less mathematically inclined than Project Euler and so, somewhat more approachable.
Referral for Honor.
http://exercism.io/ is more similar to codewars, but without all the theming and rewards stuff and just shows your progress. It has a nice command-line client for handling submissions and both automated and human feedback on each submission.