Run by the British Library, EThOS allows you to search over 400,000 doctoral theses in a variety of disciplines. Although some full texts are behind paywalls.
A multidisciplinary search engine, JURN provides you with links to various scholarly websites, articles, and journals that are all free access or open access. Specifically covering the fields of the arts, humanities, business, law, nature, science, and medicine, JURN has indexed almost 5,000 repositories to help you find exactly what you're looking for.
Functioning as a research and publishing network, ScienceOpen offers open access to more than 28 million articles in all areas of science. Although you do need to register to view the full text of the articles, registration is free. The advanced search function is highly detailed, allowing you to find precisely the research you're looking for.
The Berlin- and Boston-based company was founded in 2013 with the goal to "facilitate open and public communications between academics and to allow ideas to be judged on their merit, regardless of where they come from."
About 13,000 scientific journals are available with open-access.
If you’re looking for systematic reviews or meta-analyses, you should search the Cochrane Library. In addition to results obtained from the Cochrane Database of Systematic Reviews (CDSR), Cochrane library search results can also include control trials in MEDLINE and EMBASE, Cochrane protocols and editorials.
Cochrane library is a subscription-based search platform. However, some open-access articles may be retrieved, depending on when the article is published.