Whether its to entice users to send them $12 a month for a box of games they may or may not want, Humble Bundle are offering the original X-COM to anyone and everyone completely free of charge.
With 3 days left on the clock before this month's crate of games makes way for the next, Humble Bundle seem to be offering everyone X-COM: UFO Defense for free - potentially as a way to highlight that their offer of XCOM 2 and other assorted goods for $12 this month is about to leave them for good.
Whether it's a marketing tool or not, it doesn't deviate away from the fact that a key piece of gaming history is now free for anyone willing to take them up on their generous offering. Usually priced at a measly few dollars, you now have around 24 hours to get it for the low-low price of $0.00 - or add it to your wishlist for some reason.
Once checked out, the game will be made available to you through the usual single-use Steam code. If you don't have a Steam account already, you'd better go make one. Just don't look at the store unless you're open to the idea of paying out for games you'll likely never get around to playing.
Being such an old game at this point, they can only offer X-COM: UFO Defense as a PC title. There's likely ways around it at this point, but this Steam version isn't for your Mac or Linux machine just yet. Grab it here.