A long time ago, I built http://www.ipaidthemost.com/, which is kinda related, at least to TMDH anyhow. Far, far less collaborative than /r/place, but similar in terms of staking out ownership.
Actually this idea has been around for years, and sadly isn't new at all. I just checked and there is one that's been around since at least 2006, http://da-archive.com/index.php?showtopic=42405
Right, true. Collaborative drawing was basically the "hello world" of real-time platforms back in the day (Firebase, Parse etc). But I think most of those were ephemeral canvases.
Yeah, collaborative drawing is kind of old hat, but when you can use the context of the modern, social web to provide some new modes of interaction around it, it can be interesting again. Same applies to more mundane things like text.
Not quite the same, but my startup, Formgraph[0], also does public, real-time collaborative drawing. I even did a similar write-up about the stack behind it the other day[1]. Looks like they're relying on Cassandra and Redis. I went with RethinkDB. Should probably do a write-up about the front end in the future.
Some redditors have created /r/place derivatives already. I'm not aware of one prior to /r/place but it seems impossible that it hasn't been done before
There were definitely a few that popped up after /r/place closed down. (pxls.space being the most popular) They were nowhere near as successful as /r/place though.
That was not realtime nor collaborative on a single pane. It was a "remix" platform where one user created the initial drawing, then people used it as a base and drew around/over it to change it - in a new picture.
Also, reminds be of the million dollar front page [1].
[1] https://en.wikipedia.org/wiki/The_Million_Dollar_Homepage