https://jsfiddle.net/ used to be also very popular, I see that it is still available, but haven't used it in a while.

Which other alternatives do people use now days?

https://jsbin.com is what I've used since forever, it's fast and easy, used to use JSFiddle before that. I think you used to be able to create "bins" without having to login way back though. Honestly don't find much use for them anymore nowadays, if I want to share any web/web-like I do it via my own servers + domain and send links to people.