The World Wide Web. Yep, plenty of antecedents, I personally start with Vannevar Bush's Memex for the idea, and there were many attempts with varying levels of un-success, until Tim Berners-Lee put together a winning, massively network effects positive solution.
Which, along with search engines, have resulted in significant, qualitative changes in information publishing and access; in the '70s (or perhaps earlier) Jerry Pournelle was looking forward to the day when you could quickly find the answer to any question that had an answer, and this is one of the biggest steps towards that.
To the point I rank it near computers per se; from Wikipedia (https://en.wikipedia.org/wiki/General_purpose_technology):
General-purpose technologies (GPTs) are technologies that can affect an entire economy (usually at a national or global level). GPTs have the potential to drastically alter societies through their impact on pre-existing economic and social structures. Examples include the steam engine, railroad, interchangeable parts, electricity, electronics, material handling, mechanization, control theory (automation), the automobile, the computer, and the Internet.
This was the step that made the Internet into a "monster". So even if it only counts as one invention, it's as big as many of those preceding GPTs.
