Code Golf Stack Exchange
This site has code golfing challenges as well as other coding challenges. You can get reputation points for getting upvotes on challenges proposed or solutions submitted. Solutions are publicly submitted to the site. You can use any programming language as long as you have a way to run it online so people can test your solution.
Weekly code golfing contests are held in the contests section from Sunday to Wednesday. Piston is used to verify solutions. Solutions are revealed at the end of the contest.
Newer site with weekly problems. The top 50 solutions in each language are revealed at the end of the end of the week.
Here anyone can submit problems. Some go on forever with hidden solutions and others are timed contests where solutions are revealed after the time runs out.
Here you can compete in short Clash of Code challenges with others here or you can submit solutions on your own time here.