hackattic - No nonsense, real world programming challenges
HackerRank - HackerRank is a platform that allows companies to conduct interviews remotely to hire developers and for technical assessment purposes.
Codewars - Achieve code mastery through challenge.
Codility - Codility provides a SaaS platform with advanced validation, security and protection features to evaluate the skills of software engineers.
Real Dev - Real-life coding skill challenge
iMocha - Make intelligent talent decisions.