Steam Workshop

"Your own custom apocalypse. Discover and download new maps, Add-ons, Mutations and more! Want to try your hand at modding or want to upload your mod to Steam? Click here to learn more."

- Workshop description

The Steam Workshop is a database for uploading to user-created content available on Steam.

The only game in Left 4 Dead series with Steam Workshop support is Left 4 Dead 2.

The Workshop is split into variety sections: such as custom campaigns, weapon/skin mods, sound mods, UI mods and some scripts and other kinds of modifications. These can be downloaded by clicking "subscribe".

It's introduced after February 7, 2013 update along with most other games such as Counter-Strike: Global Offensive, Team Fortress 2 and notably Garry's Mod.

This made acquiring addons more easier since it automatically update the addon when it's updated. Unlike when manually placed by oneself. Furthermore, if a user has multiple devices the new or another device will download the addon as well when it's not downloaded yet.

On the downside it does require network connection. Trough the game usually does need connected network anyways.