Any site you know of, the info would be helpful.
A friend of mine recently came across this https://magmastudio.io/
There's also Deviantart Muro
Oekaki? I know this one: http://en.oekaki.nl/
I've used Aggie a few times to draw with friends.
There's Pixiv Sketch but you need a pixiv account for it. (Never actually used it, it doesn't work properly on my old af computer.)