CodeWars is a dojo-themed website where you can improve your skills by training with others on real code challenges, curiously named kata.
Each challenge is ranked from <8kyu> (easy, basic, fundamentals) to <1kyu> (hard, expert, compilers).
You can train on a variety of skills, including regular expressions, databases and algebra. It even has some fun puzzle exercises that test your mind's creativity to solve problems in different and unusual ways.
This repository includes my solutions to all of my completed katas.
You can export your kata solutions as well! Just clone brunolm's codewars-exporter and follow the instructions.