Unstable Build 42.13, which contains multiplayer support, has been released.

Merry Christmas. Ho ho ho.

Multiplayer is WIP and has been released for stress testing.

For the best experience you are STRONGLY advised to play on whitelisted servers or in Steam co-op.
It is currently inadvisable to have more than 20 players on a server. This will be further improved.
Please do not use debug during MP games. This will impact negatively on server performance.
Please disable all mods. Even client ones.
Ragdoll functionality has been disabled in MP. This will be added in later patches.
Please be aware that future updates to the Unstable branch WILL disrupt ongoing MP games.
Saves created before Unstable 42.13 are no longer compatible. The conversion process will very likely break elements of your game. Please back up your save before continuing, after which debug could potentially be used to restore any lost items, animals, carrying capacity and player attributes.
Much has changed under the hood. Please be patient with the modding community while they update to make their mods compatible.
Information for modders on 42.13 can be found here. We are also working to provide modders with more guidance, and game code improvements, alongside future Unstable updates.
Additional balance, QOL, and game mode / feature improvements will come in future patches.

Build 42 is currently on the Unstable beta for all Project Zomboid players on Steam.

To find it:

Right-Click game in Steam library -> Properties -> Click “Betas” tab -> Select “Unstable” from the Beta Participation Dropdown Menu

When it is clear that the version is stable enough, and likely after it has been hotfixed, the latest version of 42.13 will be uploaded to GOG.

The full changelist for 42.13 outside of MP is quite long. You can find it here. Some highlights of its contents run as follows:

Improved target selection when aiming firearms.
Improved interior lighting. Barricades (except metal sheets) and Curtains allow some light to pass through them, even when vision is blocked.
Multiple Inventory-related QOL improvements including double-clicking actions, additional equipping while walking, and auto-deactivating unequipped items.
First pass of re-organization and new icons to streamline RMC interactions
Continued Crafting/Building interaction improvements