Atom Zombie Smasher came to Linux a long time ago, but the Linux version never did make it to Steam. It is now being updated by Ethan Lee and the Linux version will be put onto Steam.
I spoke to Ethan on twitter who confirmed he was behind it:
About the game
Take command of your mercenary forces in Atom Zombie Smasher. The zombie apocalypse is upon us and it's up to you to evacuate everyone before they fall prey to the undead.
Conduct massive rescue operations with a helping hand from snipers, artillery crews, orbital bombardments, and more. Experience the tactical richness of fully-destructible environments and procedurally-generated campaigns. And when the apocalypse ends, create your own game mods and share them with the Online File Share.
For those who don't remember it, the game was released for Linux originally in January 2011. It was then added as a bonus game to Humble Indie Bundle 3, so a lot of people likely already own it.