Codechef!

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

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.

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

SPOJ (Sphere Online Judge) is an online judge system with over 315,000 registered users and over 20000 problems. The solution to problems can be submitted in over 60 languages including C, C++, Java, Python, C#, Go, Haskell, Ocaml, and F#. SPOJ has a rapidly growing problem set/tasks available for practice 24 hours/day, including many original tasks prepared by the community of expert problem setters associated with the project.

Learn to code (for free) with interactive challenges. Gain XP, unlock achievements and climb the leaderboard. It's the fastest, easiest, most addictive way to learn.

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.

At Geektastic we help our client's thoroughly evaluate their future talent using our unique peer reviewed code challenge platform. We believe a candidate is worth more than a score and the only way to thoroughly understand how suitable they are to work in your team is to have a human look over their code. The platform allows clients to license code challenges from our portfolio or upload their own, they can also choose to have our on-demand team (of exceptionally talent software engineers) carry out the thorough review or perform it in-house. »