Google Scholar
Google Scholar contains abstracts, full text and links to full text of all kind of documents with some scientific value across the disciplines.
Also check out this e-module for more information about using Google.
Just type in the title of an article to see if an accessible version is available – using quotation marks around the title if you get too many results.

Narcis
NARCIS provides access to scientific information, including (open access) publications from the repositories of all the Dutch universities, KNAW, NWO and a number of research institutes, datasets from some data archives as well as descriptions of research projects, researchers and research institutes.

Directory of Open Access Journals (DOAJ)
DOAJ offers access to over 10.000 open access journals.

Science Open
ScienceOpen contains over 37 million articles, a large part in open access.

OSF Preprints
OSF offers access to over 2 million open access preprints.