programmr

http://www.programmr.com/

ProgrammingPraxis

https://programmingpraxis.com/contents/chron/

CodeWars

http://codewars.com/

HackerRank

https://www.hackerrank.com/

Exercism

http://exercism.io/

CodeFights

https://codefights.com/

Coderbyte

http://coderbyte.com/

Cyber-Dojo

http://www.cyber-dojo.org/

CodingBat

http://codingbat.com/java

Pythonchallenge

http://pythonchallenge.com/

ProjectEuler

https://projecteuler.net/

Dailyprogrammer

https://www.reddit.com/r/dailyprogrammer

Programming Puzzles & Code Golf

https://codegolf.stackexchange.com/

CodinGame

https://www.codingame.com/

CodeAbbey

http://www.codeabbey.com/

LeetCode

https://leetcode.com/