Coderbyte 
Beginner — Intermediate
They also provide courses in: Algorithms & Data Structures, Web Development, coding bootcamps.
Codewars
Beginner — Intermediate
CodeFights
Beginner — Intermediate
CodinGame
Beginner — Intermediate
TopCoder 
Intermediate — Advanced
They also offer sponsored competitions where you can win prizes
HackerRank
Intermediate — Advanced
LeetCode 
Intermediate — Advanced
They also provide: Mock Interview section for job interview preparation, coding contests and articles
CodeChef 
Intermediate — Advanced
GeeksforGeeks 
Intermediate — Advanced
Codeforces
Advanced
Source: Daniel Borowski
Photo by Stanley Dai
Oh Lists! 