- https://codefights.com/ -- has timed problems for all skill levels, great for starting
- https://www.codewars.com/
- https://leetcode.com/
- https://www.hackerrank.com/
- https://www.topcoder.com/
- https://icpcarchive.ecs.baylor.edu/ -- website with official problems from ACM International Collegiate Programming Contest -- good for practice for that contest
- https://uva.onlinejudge.org/
- https://projecteuler.net/ - project Euler -- not coding in particular, but fantastic mathematical challenges that are intended to be solved by applying mathematical skills in code.
