Harvard University(free registration):

https://www.edx.org/

National Digital Library Project of India (free registration : developed by IIT, Kharagpur):

https://www.ndl.gov.in/ or https://ndl.iitkgp.ac.in/ to access academic  resources free of cost.

Open Yale Courses:

https://oyc.yale.edu/courses

South Asian Archive:

http://www.southasiaarchive.com/

Jstor:

https://www.jstor.org/open/

Project Muse:

https://about.muse.jhu.edu/muse/open-access-overview/

ProQuest: rich repository of humanities and social sciences:

https://pqdtopen.proquest.com/search.html

WBDSPACE: Huge resource of rare books on Sanskrit, Bengali, English, Santali,  History, Politics, Sociology, Psychology etc. :

 http://dspace.wbpublibnet.gov.in:8080/jspui/

Social Science Research Network

https://www.ssrn.com/index.cfm/en/

CORE: Humanities and social sciences eResources:

https://core.ac.uk/

ScienceOpen: for all areas of science including climate change and ageing:

https://www.scienceopen.com/

ar.Xive.org : All areas of science resources supported by Cornell University

https://arxiv.org/

Paperity: multidisciplinary eResource

https://paperity.org/

Dryad: huge resource for scientific research data

http://www.datadryad.org/

Ethos: 500,000 doctoral theses in all areas made available by British Library

https://ethos.bl.uk/Home.do

Huge resource of doctoral theses made available by Australian National University

https://openresearch-repository.anu.edu.au/handle/1885/3

DART Europe: doctoral theses from all over Europe:

http://www.dart-europe.eu/basic-search.php

Ebsco host: doctoral theses on literature, politics, international relations, law, history, sociology etc:

http://web.b.ebscohost.com/ehost/search/basic?vid=0&sid=011781a7-3b64-4269-a279-03accd843045%40sessionmgr120

SemanticScholar: science-related resource

https://www.semanticscholar.org/?gclid=Cj0KEQiAkO7CBRDeqJ_ahuiPrtEBEiQAbYupJfG10GEbuSyABnQkt3G-wMpzMcw1Q01zzAr3aOvl8-QaAtUr8P8HAQ

DLC: supported by Indiana University

http://dlc.dlib.indiana.edu/dlc/

BASE: interdisciplinary resources provided by Bielefeld University, Germany

https://www.base-search.net/

Trove: resources for digitised Australian newspapers (since 19th century), govt gazettes, books, journals, maps etc provided by National Library of Australia :

https://trove.nla.gov.au/