Spoj - Sphere Online Judge created for the coders' community to train, to learn and to make progress.

hackattic - No nonsense, real world programming challenges

Codewars - Achieve code mastery through challenge.

HackerRank - HackerRank is a platform that allows companies to conduct interviews remotely to hire developers and for technical assessment purposes.

LeetCode - Practice and level up your development skills and prepare for technical interviews.

Takehome - Effective and humane coding challenges for candidates