When a Redditor by the name of Crruzi decided that he wanted to take his newly-acquired Visual Basic for Applications (VBA) skills out for a spin, he did what any of us would do: He created EXLCOM, a follow-up to XCOM: Enemy Unknown that runs in Microsoft Excel.
Yes, that's Excel as in the famous spreadsheet software created by Microsoft, and no, it's not at all actually something most people would do. But Crruzi is a big fan of XCOM, and this struck him as a natural way to indulge that fandom while simultaneously increasing his familiarity with VBA.
"Two months ago I couldn't write a single line of code either—just keep at it," he encouraged another Redditor who's in the midst of learning VBA. "I feel like excel is a great (but also often times infuriating) environment to learn to create simple programs in, because you can use it quite easily for a wide range of real-world issues, and it is often more powerful than one might expect."
EXLCOM is set between the events of Enemy Within and the upcoming XCOM 2, which you might say is a bit narratively iffy (if you want to be that way about it) since Firaxis has previously explained that the two games don't share the same timeline. However you want to describe it, EXLCOM is neither a remake of the previous game nor a representation of the upcoming one—or, more accurately, will be neither, since at this point only the tactical portion of the game is actually operational.
Recreating the combat element of XCOM in a spreadsheet would be enough of an accomplishment to keep me satisfied for a month, but Crruzi is continuing to work on the project in order to incorporate the strategic layer as well. (For now, character stats and weapons are randomly assigned.) It's obviously not as pretty as the more recent XCOMs, or even the originals, but according to PCGamesN, "it functions pretty much identically" to the Firaxis games.
And it runs in a spreadsheet! That alone is worth the price of admission, which by the way is free: Crruzi warned that "it will definitely not be a polished experience" right now, but if you don't mind dealing with some bugs and balance issues, you can snag a copy of EXLCOM to try for yourself here.