Microsoft Flight Simulator 4. The first of the series to be openly modifiable (sort of…), expandable and it even had full colour solid graphics, which was still something of a big deal in 1989.
Well the Internet Archive has a treat in store for you if you’ve never tried this sim; not only the ability to run it on a modern PC, but in a web browser, no less. Click here to look back in time
15 25 years* – but you may want to turn your sound down a bit. 🙂