Library Genesis / LibGen
Based in Russia, this is the largest currently openly available collection. They have several initiatives: i. 1 million files of mainly non-fiction ebooks, ii. 900k of mainly fiction ebooks, iii. 20 million papers from journals of science, history, art etc., iv. comics, magazines and paintings; totally amounting to at least 30 TB — easily the Library of Congress of the digital world. The Libgen movement mainly focusses on downloading whole collections that have already been posted in fora or as torrents, although a significant portion is also due to individual efforts.