But only if you're quick - yes, I missed this one, but Ashes of the Singularity: Escalation is currently free on the Humble Store if you sign up to their newsletter, while stocks last, and through until tomorrow.
It's a real-time strategy game developed by Oxide Games and Stardock Entertainment (yes, those people who used to make WindowBlinds) - and it's only 4 years old, so to grab it for free is a stonkingly good deal. It mostly scored average amongst critics and gamers, but hey, as I always say, free is free!
If you're in time, you can grab the game for free by clicking here. As mentioned, you need to sign up for the Humble Newsletter, but you can always unsubscribe afterward. Make sure you claim the key too, as they expire on the 14th of May if you don't.
Humble also have their Spring Sale on right now with up to 90% off over 4,000 titles, so it's worth heading on over to see if you can grab yourself a bargain regardless.