I would also say that the blooming "bookz" (ebook scanning) community around a decade ago was instrumental in making much knowledge accessible, especially to those in poor or developing countries who wouldn't be able to afford even a single book or access to a library with them.
Now, LibGen serves a similar purpose (but there's lots of stuff elsewhere that it doesn't have either...)
Everything else is scattered over the rest of the Internet, including archive.org and various more specialised forums (e.g. if you are looking for service manuals for farm equipment, petroleum industry machinery, etc.)
That said, whenever I do find something rare and useful, I upload a copy to LibGen too.
Now, LibGen serves a similar purpose (but there's lots of stuff elsewhere that it doesn't have either...)