Agreed. http://gen.lib.rus.ec is my go-to. If it ever goes down then a quick web search or a visit to https://en.wikipedia.org/wiki/Library_Genesis and the new mirror appears.

Also, literally everyone already knows this, but SciHub for journal articles: https://sci-hub.tw (mirrors: https://en.wikipedia.org/wiki/Sci-Hub).

Libgen is incredible if you need a chapter or two each from a whole bunch of books that’d qualify as obscure outside of a university library. Or if you want the TOCs and indices from a huge number of books of that sort so you can figure out which ones you even need, and when. Massive time saver.

I mean it’s great for other things too but for that kind of dragnet research—damn is it nice.

[edit] oh and for when you’re just getting started and just need the introduction and sources from a representative work on a topic so you can track down the foundational works. Beats the hell out of waiting on inter-library loan when you just need to skim a few pages.