Codeforces (also has SGU under acmsguru)
Atcoder (also has JOI problems)
DMOJ (IOI and ICPC problems)
Kattis (ICPC problems)
Advent of Code
Timus
Project Euler
dunjudge.me
UVa Online Judge/Online Judge
Topcoder
CodeChef
oj.uz (IOI and ICPC)
acmicpc.net (ICPC problems like Live Archive)
USACO
Leetcode
Hackerrank
e-olymp (olympiad)
toph.co
acmp.ru
judtge.org
URI Online Judge (ICPC)
SPOJ
PEG (migrated to DMOJ)
CSES