Today only, you can pick up the prelude to the ever popular X-COM: Enemy Unknown and X-COM 2 absolutely free via Humble Bundle. While X-COM: UFO Defense is notably a little more difficult than it's predecessors, many note that the experience is just as good as it's popular sequels. If you're looking for a breakdown of differences, this video guide should give you an idea of what you're getting into...
The deal ends at the end of the day so make sure you click the link here and see if it's still active! Also, if you're feeling frisky, you can throw down a couple of bucks and get X-COM 2 on a discount as well.