There are a range of digital tools and services which can aid the discovery and access to full-text versions of scholarly papers (and sometimes other outputs). None on their own are entirely comprehensive, so it is worthwhile trying more than one if you are unsuccessful in your first search.  Please see below for details.  NB. Sci-Hub and similar services are illegal and should not be used.

Another option, particularly for journal articles, is to contact the corresponding author to ask if they would share a copy of the final version of their paper. Most publishers permit this kind of scholarly one-on-one sharing. (Tip: an author’s affiliation and contact details may have changed between publication and when you want to read it. So, if you opt for this route, check the details on the published paper are still current).

Google Scholar is a multidisciplinary database which allows a researcher to search for full-text versions of academic papers. Please note that not all research outputs are indexed by Google Scholar.

NB. To see SFX links to available resources when you are working off campus, within Google Scholar go to Settings - Library links, and select King's College London.

Unpaywall is a database of accessible scholarly articles. It harvests open access content from many sources, matching documents to article DOIs. When you have the Unpaywall browser extension installed and view the landing page of a journal article you are interested in accessing, it will indicate whether any open access versions of that paper are available. From here you can access available full-text versions with one click. 

CORE is the largest global aggregator of open access papers. When you have the CORE Discovery browser extension installed and view the landing page of a journal article you are interested in accessing, it will indicate whether any open access versions of that paper are available. From here you can access available full-text versions with one click. 

This extension can be added to most web browsers to help you find a wide range of open access resources. It becomes active when your web browser is directed at a journal article. When you use the Open Access Button, it will either take you straight to a free copy of the research article or help you ask the author to freely share the article with you.

PMC is a repository which provides access to full-text versions of publications in the PubMed database.  It covers the fields of biomedicine and life sciences.

The Library Search open access button indicates resources that are available OA, with the proviso that - the open access indication appears only on records that are flagged as open access in the index. In cases where a group contains both open access content and restricted content, the represented item in the group may be restricted, but other records in the group may be flagged for open access after the user clicks the All Versions link.