GOG.com are offering Xenonauts for free until June 6, 2018 at 1 PM UTC. Their version is compatible with the Windows, OS X and Linux operating systems (please see their product page for the full system requirements).
Xenonauts is a strategy game that normally costs $24.99. The GOG version also includes two wallpapers, a quick start guide, a soundtrack and seven avatars.
To take advantage of this deal, simply click on the "GET FOR FREE" link on the top of their homepage (assuming you're already logged into their store).
Image credit: GOG.com.