Prison Architect, the engrossing prison building sim from Introversion had a big update out with multiplayer now done and out for all.
However, there's some caveats. The multiplayer mode does not yet support Escape Mode, Warded Mode and any prisons that require Mods.
Update explanation video:
A very fun game, I've yet to try the multiplayer but I can imagine it being very interesting considering how good it is when simply building by yourself. I didn't expect it to be supported so well after release with new features too, Introversion are a developer worth supporting.