Prison Architect has a little present for everyone before Introversion Software go into holiday lockdown. First debuted in September as a special opt-in build, the carceral management sim's online multiplayer mode is now polished enough to be included in the game proper. Online mode is still considered 'alpha' and has some limitations, but they've otherwise got the core of the tiny-person punishment game working online. Probably best not to run an entirely public server - this one's best with friends, for obvious reasons. Check out the developer announcement video below.
There's a few things that you can't do online. Escape mode and Warden mode are out, and mod support isn't included yet, but the developers say if multiplayer catches on they may invest the time in making those extras work too. The host player also needs a fairly robust upstream - 100 kilobytes per second at least, per player. My old broadband setup would have choked trying to do that, but anyone who streams video on any kind of regular basis should be able to handle it. You're also able to take your regular offline game and take it online to play with friends, so long as it's untainted by mods.
There's also very little in the way of communication options built in, assuming that you'll just get your buddies on Discord or Steam's built-in voice chat. Given that running a public-facing server is just asking for strangers to draw giant dicks all over your jail, you'll most likely be running a private game with friends, so it's not an unfair assumption. While obviously not the best fleshed-out multiplayer mode, this seems like a good fit for the kind of people who casually stream themselves playing the game - there's a few of them on Twitch at any given time. While the subject matter makes it a little weird to turn into a casual social thing, I can't fault Introversion for going that extra (green) mile.
Prison Architect's multiplayer mode is out now in the main build of the game, no opt-in necessary.