Be sure to check out Alien Swarm: Reactive Drop. It expands upon Alien Swarm, bringing you everything you already love, plus a whole lot more!

Reactive Drop's Source Code is available for you to make your own additions to the game.

Having access to the source code you can:
⠀• Add new weapons, NPCs, fix bugs and generally add new features to the game and submit them
⠀⠀as pull requests
⠀• Create mods. Both server side and client side mods. Or fully standalone mods

Download it from GitHub [github.com]