Untrusted: https://alexnisnevich.github.io/untrusted/

http://www.codeandconquer.co/ is vaporware at this stage.

https://www.battlecode.org/ is also great fun each year. Not sure if it continues running after the competition is over.

http://aichallenge.org/ hasn't run either for awhile unfortunately.

It's the best one for doing challenges that use actual test cases. This challenge could easily be ported to that site (and I think there already are a few elevator ones)

It abstracts simple programming concepts (like loops, branches, etc) in a visual language.