XCOM: UFO Defense is not the turn-based combat strategy game that defines the genre. In my mind, that honor goes to the sequel, XCOM 2. But for 5 more hours as of writing this article you can get that precursor to the masterpiece for the low low cost of absolutely nothing. The Humble Store is running a promotion but time is nearly out.
The free copy is an activation code for Steam. XCOM: UFO Defense was originally released in 1993, and has been rebranded and re-released over the years, going so far as to land on Android and other platforms as the reimagined XCOM: Enemy Unknown. This copy, however is for the original in all it's 25 year old glory. The promotion is set to end around noon Eastern time.
Here's the direct link for that free copy of XCOM: UFO Defense. The clock is ticking: https://www.humblebundle.com/store/xcom-ufo-defense-free-game