Insurgency, a shooter with roots as far back as the days of Half-Life 2 mods, is free for a limited time on Steam. If you download and play the game now, which costs you nothing, you own it forever.
The game was released in 2014 by New World Interactive, acting as a sequel to standalone HL2 mod Insurgency: Modern Infantry Combat. The game is likely going free to help advertise Insurgency: Sandstorm, which is coming out next month.
You can only download Insurgency for free for 48 hours, so get to it quick. You have to make sure to launch the game at least once to keep it, which means downloading it first.