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)
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.
This. I suppose you could argue it's also a source of enormous human frustration and negative impact, but fundamentally Linux changed how software worked as a whole. Server operating systems quit being a business, which commoditized the concept of owning a server to it's logical extreme. It ushered in a panoply of suitable software replacements for expensive things like C compilers and web servers, which would eventually whither away as viable products entirely.
At the expense of killing a few exploitative businesses, Linux created a better world for millions of programmers and the hundreds of millions of users that rely on the web staying free. It's hard to imagine an alternate reality where Linux doesn't exist but the internet does - we'd be relying on the BSD-likes to not get forked into commercial systems, which is a pretty unstable premise.
Seeing as how the internet, the world wide web, and the GNU software ecosystem had all existed prior to the development of the Linux kernel, I can easily imagine the internet existing without the Linux kernel.
As you point out, BSD had existed since 1978.
Even so, you can't get the Linux kernel without a C compiler - which gets my vote! :)
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.
