Variation Simulation Analysis Software.
It's a technique for simulating variations in product assemblies. Usually mechanical, but could be of other natures, as well. You model the assembly and it's manufacturing variations, and then "build" some quantity of parts. One can determine how many assemblies will likely meet specifications, the major contributors to out-of-spec assemblies, etc. etc.
The technique was developed during WWII at Willow Run Labs, where it was implemented by the classic "banks of women operating calculators", and is one of the reasons we were able to crank-out all those airplanes that actually worked.
By the 70's it was implemented in an academic setting on mainframes.
A company I worked for obtained rights to VSAS and we ported it to the IBM PC. I did the initial port to Watcom Fortran (there's another one for you!), and then designed a domain-specific language (VSL) and implemented a compiler in C and interpreter in Fortran, so that mechanical engineers didn't have to write their models in Fortran any more. The Fortran models were bulky - with line after line of function calls with zillions of parameters, passing separate X,Y,Z values in the calls. I'd imagine the engineers wore-out the parenthesis keys on their keyboard pretty fast. VSL, on the other hand, had data types for points, lines, vectors, planes, etc. Using an interpreter didn't slow things down, because most of the time was spent in geometric library routines, which were in carefully-optimized Fortran.
I insisted on their hiring a mathematician, and between the two of us, we tweaked it to run faster on the PC than it did on the mainframe. (Engineering professors don't write code that is either fast or mathematically-correct, it turned out...)
And that's when it's use took off. The company founder started as a manufacturer's rep for some Finite Element Modelliing software, so had lots of contacts in the auto industry. (And the company was located near Detroit.) They both sold the software and did also did in-house projects for the auto companies until they ramped-up their own engineers. This allowed the auto makers, for example, to start treating windshields as structural elements (because the hole for the windshield could be manufacturered to precise tolerances), and allowed them to eliminate costly alignment operations, such as when fitting hoods.
It's used by every auto and aircraft manufacturer, every hard disk manufacturer, etc. etc. etc. Basically just about any complex mechanical product you touch was touched by VSAS during design.
I'd imagine you couldn't build an iPhone at an affordable cost or with the quality level of an iPhone without VSAS (or it's equivalent). You wouldn't be able to buy a terabyte hard drive for less than $100.
There's more info on it here:
(The company was acquired by Siemens many years ago.)
Maybe not quite what this post was looking for, which I think was more consumer PC software. But it runs on a PC and has from the beginning of PCs, and has had a large but mostly-invisible influence on just about every tech product we use every day.
A 30-year run is nothing to sniff at, either.