Xenonauts is a fairly hardcore strategy game that will have you use small squads of persistent soldiers to fend off a massive alien assault. If this sounds exactly like XCOM to you, that would be because Xenonauts takes heavy, heavy inspiration from the original UFO: Enemy Unknown. It's still its own game with its own unique quirks, but you can definitely see XCOM's influence all over it.

If you feel like this is something you might enjoy, you should hurry on over to GOG as Xenonauts is completely free to grab for the next ~9 hours. Simply go through the whole 'purchasing' process, claim your copy, and you'll be able to play it at any time you want. You don't even need to install it right now, just make sure you claim it before the deadline is up.

As far as its quality is concerned, I would say that Xenonauts is a bit of a mixed bag. It's definitely faithful to the oldschool XCOM game and it has many of the same charms, but it also has a lot of the same annoyances and UI problems that make the original XCOM a bit of a chore to play these days. In other words, I doubt Xenonauts will appeal to you if you're not a big fan of strategy games, but if you do have a bit of a soft spot for the original XCOM then I would definitely recommend you give Xenonauts a try. After all, it's completely free!

And just to give you a bit of an idea of what you're getting yourself into, I'll leave you with the GOG trailer. Enjoy!