The COBOL compiler. I started life coding in assembler, and COBOL made it possible to write programs and functions and not just code.
Regards,
Shodan, Dinosaur
Definitely Visicalc. " It is often considered the application that turned the microcomputer from a hobby for computer enthusiasts into a serious business tool, prompting IBM to introduce the IBM PC two years later.[2] VisiCalc is considered the Apple II’s killer app"
It was superseded by Lotus, and then Excel.
Curse you, TriPolar, for beating me on one of my nominations! But I’m going to plow ahead anyway and say what I was going to say about an hour ago before I got distracted by something else … 
I’m going to nominate three window-oriented GUIs:
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The Xerox Alto executive, Exec, for the reasons already stated. It was the very beginning of what became both Microsoft Windows and the Mac operating systems after Apple and Microsoft both stole the idea from Xerox. This has special meaning to me because I was actually there at Xerox PARC during that magical time when the windows GUI, the PC, the mouse, the laser printer, and the Ethernet LAN were being developed before anyone had ever heard of – or even thought about – such things. I was just there as a visitor, sadly, as I was just a kid, but still, I was there, saw and touched the stuff, and breathed the same air as the exalted beings who invented personal computing as we know it nearly half a century ago. All these years later I still remember standing outside on Coyote Hill Road and just going, “Wow!”
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Microsoft Windows 2.0 – the progenitor of the OS we all love to hate. Reason for nomination is because it transformed the computer industry, the PC interface paradigm, and in the process, pretty much started the transformation of the world. I suppose there was probably a 1.0, but knowing Microsoft, it probably didn’t work,
so I’ll go with 2.0, which I know worked because I actually used it for a while on a primitive 286 PC, although it was advanced enough to even have a 10 MB hard drive (the drive enclosure was about the size of a pizza box and the computer sat on top of it). Windows 2.0 taught me amazing things like “how to use a mouse”! -
Windows XP. Reason for nomination is that Windows XP was the first full integration of all the consumer features of the previous consumer versions of Windows with the elegant and bulletproof Windows NT kernel from the business and scientific versions. Simply put, Windows XP was the first consumer version of Windows that didn’t crash if you looked at it sideways, especially when gaming. It had all the consumer features of previous versions of Windows, and more, yet the kernel was super stable, hardware protected, and just never crashed, unless you had a buggy kernel component that you added yourself, like a buggy driver. As such, Windows XP marked the end of Microsoft’s dual line of consumer versus business/scientific operating systems. From then on, there was only one Windows. It was revolutionary, and in my view, relative to Windows XP, all future versions of Windows were essentially incremental changes, many of them ill-advised and regressive.
It might be stretching even the generous rules of the OP, but I’d argue the TCP/IP suite of protocols. An enormous part of our lives depends on it.
And 95 had the coolest startup chime, as far as I’m concerned. (And, yes, I know it was composed by Brian Eno, but I didn’t know that until well after I stopped using Windows 95.) Then again, after Windows 3.1’s annoying brash TA DA!!! startup, anything was an improvement.
The two pieces of software that make the Internet possible.
Firstly the server software that allow packets of data to be sent along a network. It started with ARPANET.
Secondly, the web browser. The very first one, Nexus(text only) or Mosaic 2.0 (the first with graphics)
I am a generous OP and I accept your nomination. 
I remember win 3.1 for Workgroups didn’t include Tcp/IP
My employer had a site license for the Tcp/IP software.
I easily installed at least 40 copies of it and then configured the network.
It was a relief when Win 95 included Tcp/IP.
For games, I may be biased since it was one of my first games but I’d put in a word for “Ultima 1” one of the first role playing D&D style adventure game with quests to be completed, secrets discovered, experience and gold to be acquired and final boss to be defeated. Also included dungeon exploration with first person perspective 12 years before wolfenstein 3D. I realize that Akalabeth preceded it but Ultima 1 was what really got adventure role playing started.For games, I may be biased since it was one of my first games but I’d put in a word for “Ultima 1” one of the first role playing D&D style adventure game with quests to be completed, secrets discovered, experience and gold to be acquired and final boss to be defeated. Also included dungeon exploration with first person perspective 12 years before wolfenstein 3D. I realize that Akalabeth preceded it but Ultima 1 was what really got adventure role playing started.
I distinctly remember in 1980 the power and satisfaction as a 10 year old of walking around a computer show room and typing
10 PRINT "HELLO "
20 GOTO 10
on all the computers
My own little bit of graffiti and vandalism.
The spreadsheet was the PC’s killer app. IMHO, not even an operating system or all early video games match its importance. Excel and is kin showed normal people what a computer could really allow you to accomplish, in a business setting, in a way that was visible and relatable.
What about the MS Flight Simulator from back in the day? I once found that it was the basis for “compatibility” in that computers that could run it meant that they were _________-compatible. I offer it as a suggestion for the forced normalization of computers, languages, and communication. Even today, Apple files are legible on a Windows and Unix machines and vice-versa.
Would it be too simple to say “AOL”?
They were the first to really bring internet access mainstream and you could argue the internet is the greatest invention in mankind save for writing and math (and I think we had a thread recently arguing those…)
In terms of installed base, the various *nix’s have Windows beat by at least 2:1
Since both iOS and Android are based on some flavor of Unix (FreeBSD for iOS, Linux for Android), I’d say that Unix and Unix-like OSs are the clear dominant software app for the purposes of this thread.
Hunt the wumpus.
I would say, whatever the first interpreter/compiler was - the days of having to flip switches to code were pretty much over at that point.
Google’s search algorithm
This. The internet runs on 'nix.
While the code that built the internet might be more crucial, I’d got with Mosaic as the thing that made owning a computer a necessity not a toy or luxury. Before it came out there was some concern that the PC revolution had burned itself out, and that there weren’t a lot of reasons for people to buy PCs at home except keeping recipes, playing games and some word processing. After Mosaic you started seeing urls on the side of buses.
I say this as someone who used Unix System V, Archie and Veronica, and various minicomputers.
Fortran predated COBOL, as was the existence proof that a high level language was practical.
The first assembler, as far as I’m aware, was written by Don Gilles a student of von Neuman’s at Princeton, for the IAS machine. No cite, but he told me this himself. So they had stopped flipping switches long before FORTRAN, though I flipped some switches myself to load the beginning of the boot routine into location 0 of one of our PDP-11/20s. The other had a ROM with the boot routine in it.