What are the best online websites to get access to free academic resources (references, articles, books, etc..)?

for books , gen.lib.rus.ec is good. for articles, try sci hub. either way you are not doing a good job. every book is written with blood and sweat. every article involves hard work. he or she must be rewarded . payment is one way. when you hack, it is injustice. i will not suggest this. people or companies deserve something for their effort. 

  for me it depend on the topic and you can search through using the key word and there is some free access website but in general i use google scholar 

I guess, no one will be able to anwser this question since we do not know what scientific field you're specialized in