X-COM: UFO Defense is free on Humble Bundle for a limited time. The classic DOS game from MicroProse Software, is also called UFO: Enemy Unknown.
The game is set in a sci-fi theme, and includes a combination of RPG, 4X elements along with turn-based strategical combat.
The gameplay is unique, in that you have to shoot down UFOs and then investigate the crash site of the alien craft, and battle with the surviving aliens. There are a variety of weapons and vehicles at your disposal, and you will even be able to research Alien tech which you recover, to make your own powerful weapons and use them to save the Earth.
Sure the graphics are not great now, but the game is from 1993. And the fact that X-COM: UFO Defense is one of the few games on Steam, which boast a 10/10 rating, and has an overwhelmingly positive review score, should tell you how good the game is.
How to get XCOM UFO Defense for free:
1. Visit the Humble Bundle page, and click on the "Get it free" button. (You have to be logged in to your account.)
2. The button will change to checkout, and you can complete the checkout process.
3. Check your registered email's inbox for a mail and you will get a link, which you can use to claim the game (adds it to your library).
4. Now, click on the "Redeem on Steam" button (Requires you to link your Humble Bundle account, to your Steam account).
5. Copy the key and open the Steam client, and click on the "+ Add a game" button on the bottom left.
6. Click on "Activate a product on Steam", and then on "Next", and on "I agree", and paste the key here.
7. Lastly, select next and you will have added the game to your Steam library. Click on the finish button to close the pop-up.
You can download XCOM UFO Defense from your Steam library. You have 1 day and 10 hours to get the game for free. But do remember to redeem the key on Steam before 10:00 AM PST on February 15th.