What's the most positively impactful software ever written?
What piece of software has brought about the most human happiness? Let's ignore examples that lack counterfactual support, e.g. 'the software that ended WWII prevented x'. My intuition points to medicine but I have no concrete example.
Wikipedia is a first-blush guess. It was software to allow immense distributed human knowledge from non-technical (or lightly-technical) people to all people.
Probably the software which lets you stay in contact with your mom or girlfriend/boyfriend, friends etc. And for many people on Earth that's WhatsApp or whatever else messenger service you use. (yes, I ignore all the layers below that)
The browser. Because happiness varies but can be delivered in all its software varieties by that same, universal platform.
The rise of the browser and JS is the most significant event in computing history since the mainframe.
The C Compiler
Nearly all microcomputer software that's ever been run was built on system foundations built by the C compiler, and/or an OS kernel built by the C compiler.
Anything that was certified as 100% Pure Java
Whatever software that allows distributing (and watching) porn over computer networks. It pushes technology further, to cause more happiness. And it causes happiness because it pushes technology further (and because of 1 more thing). The perfect circle.
VLC media player. As a kid it was indistinguishable from magic.