Spoj!

Project Euler is a series of challenging mathematical/computer programming problems that will require more than just mathematical insights to solve. Although mathematics will help you arrive at elegant and efficient methods, the use of a computer and programming skills will be required to solve most problems.

A fun social platform for hackers to solve interesting puzzles, build quick hacks, code game bots and collaborate to solve real-world challenges.

Duel with friends and foes in head-to-head programming battles!

CodinGame is a challenge-based training platform for programmers where you can play with the hottest programming topics.

Coderbyte is a place built for anyone to practice and perfect their programming skills. Challenges are added almost every day so you can work on applying your algorithmic thought process on any problem you want at anytime. Coderbyte is built on improving your ability to work on common logic, mathematical, and algorithmic problems you might one day encounter.

CodeChef was created by Directi as a way to continuously challenge and engage the developer community. Our goals are to provide a platform for practice, competition and improvement as well as enable developers to benchmark their skills against their peers. We’re having fun with the site and hope you will too.

Achieve code mastery through challenge.Improve your skills by training with others on real code challenges

Develop your coding skills and practice them by competing with hundreds of players worldwide. Code your own bots in your preferred language and join the various competitions that we offer. Learn, win prizes, but most importantly, have fun coding! Check out the our current competitions to see how you could get started.

Halite is an artificial intelligence programming challenge.

Topcoder gathers the world’s experts in design, development and data science to work on interesting and challenging problems. Members are provided with opportunities to demonstrate their expertise, improve their skills, and win cash, while helping real world organizations solve real world problems.

Built by CodeFights, Code Arcade challenges players to solve programming tasks to complete game levels. It's a fun way to hone coding skills.