Looks like CodePen is making progress on being a Glitch competitor:
(All I know so far is what that post says. And I did check with the Glitch folks – as I expected, they’re very happy for us to talk about Glitch competitors here! Thanks, y’all!)
PS: I just searched for other posts about competitors and see I myself started one almost a year ago!
(I’ll wikify that now and add CodePen and remove Nitrous (RIP)!)