Unpaywall - a browser plug-in which works with Chrome and Firefox to instantly find free, Open Access versions of articles or book chapters.
Open Access Button - find free (and legal!) research articles delivered instantly or requested from authors. Also comes with a browser plug-in.
Lean Library - a browser plug-in which finds online versions of resources, including those which are Open Access.
Apollo - Cambridge's institutional repository, which containe open access versions of articles and book chapters written by Cambridge academics, including doctoral theses.
DOAB - a directory of open access books
DOAJ - a directory of open access journals
OAPEN - freely accessible academic books, particularly in the area of Humanities and Social Sciences.
Open Library of Humanities (OLH) - free access to articles in openly published journals, mostly focused on Humanities.
CELT - a free online database focusing on Irish literature, politics and history.