Here are some links to open directories I deem interesting or deemed interesting at some point for some reason or another. This list is not maintained; expect dead links.
https://the-eye.eu/
http://dl.booktolearn.com/
http://erewhon.superkuh.com/library/
https://drive.google.com/drive/folders/0BwJdvQM0LylNQW9oMldpY2Q5bUE
https://mirror.thelifeofkenneth.com/sites/
https://mirror.thelifeofkenneth.com/lib/
https://mirror.thelifeofkenneth.com/lib/World%20Tracker%20Library/worldtracker.org/media/library/
http://mirror.macintosharchive.org/
https://c64.archive.computer/
https://porn.jules-aubert.info/humble_bundle/
http://repo.palkeo.com/repositories/
https://cdn.media.ccc.de/
https://repo.zenk-security.com/
http://kunalsdatabase.com/ebooks/2015/
http://kunalsdatabase.com/ebooks/2016/
http://kunalsdatabase.com/ebooks/literature/
http://kunalsdatabase.com/ebooks/computer_ebooks/
And more subdirectories of http://kunalsdatabase.com/ebooks/