Academic Index.net
Academic Journals.org
A Miner
BASE - Bielefeld Academic Search Engine
De Gruyter Open
Directory of Open Access Books
Directory of Open Access Journals
Elementa: Science of the Anthropocene
Gapminder
Google Scholar
Ingenta Connect
Journal Seek
JURN
MDPI: Multidisciplinary Digital Publishing Institute
Open Access Library
Open Grey - Grey Literature
RefSeek
Springer Open Journals
SSRN: Social Science Research Network