AtCoder 2026-01-30
Saw this one mentioned on some Project Euler forum threads.
Kaggle Updated 2026-01-30
To be fair, this is one of the least worse ones.
LeetCode Updated 2025-07-16
Exercism Created 2025-03-20 Updated 2025-07-16
SPOJ 2026-01-30
UVa Online Judge Created 2025-03-20 Updated 2026-01-30
License: problem setter dependant, they appear to collect from past competitions.