There are several search engines that include Open Access Content, and are geared towards Open Access discovery. These include:
Google Scholar. Google Scholar has much that is full text and available to anyone - indicated by the [DOC], [PDF], or [HTML] on the results list. Click on All versions under an article citation which may show a full text freely available version also.
CORE. CORE is the "world's largest collection of Open Access research papers".
Dimensions. Dimensions contains over 100 million publications, including books, articles and research.
Open Access Button. The Open Access Button is a website and browser plug-in that will automatically search for Open Access versions of papers and articles you want to find.
For help searching for Open Access resources for a project, please see the Search Open Access website from Cambridge University Libraries.