These resources aren't topic-specific, and they're incredibly comprehensive. They're a good place to start for any subject.
|The Mason OER Metafinder||The Mason OER Metafinder helps you find open educational materials with a search across 21 different sources of open educational sites.|
|Free Ebook Foundation||Home to more than 100 free science books and more than 2,000 free programming books|
|Directory of Open Access Books||Home to more than 30,000 academic open-access peer-reviewed books, though it can be slightly overwhelming to navigate|
|Awesome Lists||Sort of like a clickable Wikipedia, linking to almost every learning resource with just a few clicks|
|Wikipedia List of Digital Library Projects||A list of online bodies of knowledge|
|Wikipedia's Psychology and Neuroscience categories||A starting point for everything on Wikipedia about psychology and neuroscience|