I have met an accountant with no knowledge in programming that wrote an entire accounting suite in an excel spreadsheet, which he would copy for each client.

You would be surprised how many accountants still do that. Accountants are a finicky bunch and they don't trust what a cloud accounting platform such as Xero/QB spit out, so they end up writing their own "custom software" in excel.

"Still"? I met him like three years ago. And the Israeli Xero/QB hasn't changed in like 20 years.

People have done some absolutely crazy things with Excel as well. To some people its like an OS.

Wasn't there a flight simulator or a Doom a decade ago?

Excel had a flight simulator as an easter egg, but it wasn't written in Excel formulas or VBA :)

Excel 97 had a sort of flight simulator. Excel 2000 had a car game. Earlier versions had other little easter eggs, but I think that Microsoft cracked down on them after Excel 2000.

Turns out the federal government to which Microsoft sells wondered, “if they’re sneaking in flight simulators and dungeons, what else are they sneaking in?” That’s the version I was told, anyway.

That, and Easter eggs are another attack vector. How much of a security audit do you think those get?

Indeed - some people have implemented games in it: http://mashable.com/2014/08/27/microsoft-excel-games/

My favorite is the Puerto Rico Evolver ( http://www.tropiceuro.com/puerto-rico-evolver/ ), which both allows you to play (vanilla) Puerto Rico (the board game), AND evolves to your playstyle as you play. Pretty amazing.