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 the Left 4 Dead series with Steam Workshop support is Left 4 Dead 2.

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

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

This made acquiring addons easier since it automatically updates the addon when needed, unlike manually placing one. Furthermore, if a user has multiple devices, it  will download the mod as well.

On the downside, it does require a constant network connection to use the addons, though the game does not need a connection to play offline.