Tripwire's hit survival-horror online co-op FPS, Killing Floor, is free for limited time on Humble Bundle. It can only be claimed for a little more than one and a half day (i.e. 40+ hours) from now on. So it's best to grab it while you still can.
To get it, login to your Humble Store account. Then add it to your cart and click "Checkout". There you need to click on the blue "Get it for free!". And it's done!