Valve's co-op shooter was already a great freebie, but now you're getting even more game thrown at you.
Community feedback revealed that the most requested additions are more variety and a greater difficulty, to which Valve has responded by adding a new Onslaught mode which introduces an AI Director that dynamically generates swarms of aliens. Onslaught works with any difficulty setting and ensures no mission plays the same way twice.
Valve has also added a new Brutal difficulty level for those who thought Insane wasn't enough of a challenge. Valve says no one on the Alien Swarm team has managed to complete a mission on this setting.
If you want even more of a challenge then you can now also toggle on Hardcore Friendly Fire, making friendly fire deal full damage immediately, flamethrowers set teammates on fire much faster, and turrets will hurt you if get caught in their crossfire.
As a reward there's also three new IAF Promotion Ranks: Platinum Star, Osmium Star, and the coveted Iridium Medallion.