Project Zomboid, arguably the best Zombie Survival game ever made -- if you like brutal, intense, deep gameplay over high-speed action -- has finally added multiplayer, again. It was dropped as part of the transition to a new, better looking version of the game and needed a lot of work to get it up and running again, but now it's here, it's back, and it's an exciting addition to a game that has been in early access for over eight years.
Project Zomboid has been in development since before 2011, where it was first released as an early test alpha. It then hit Steam Early Access in 2013, and has been continually evolving ever since. It uses an isometric perspective to offer an open world with a city and surrounding landscape to explore. Every room can be entered, looted, barricaded, and developed into a fully fledged fortress -- if you can survive long enough.
There's a very deep crafting system, a complicated health system, and now, the ability to enjoy it all with your friends. Multiplayer servers are open to the public on the test branch of the game, or you can run your own private servers (which is probably the better option for now).
Multiplayer supports up to 16 players at a time, and there's been a big update to the map to improve the scale of where you can explore and survive (and die) together. In future builds the developers hope to support 32 players on the same server. They're also still working on bringing back NPCs and adding animals, but those are still far in the future for now.