Crossref is an official Digital Object Identifier Registration Agency of the International DOI Foundation.  Every article published in USEFUL online journal is assigned with Crossref. Full metadata and open references registered in Crossref make research outputs easy to find, cite, link, and assess in any project world-wide.*Crossref (USEFUL online journal)
*Crossref (DSpace at USEFUL.academy - repository)
Also, you can see how good is our metadata.
BASE is one of the world's most voluminous search engines especially for academic web resources. BASE provides more than 120 million documents from more than 6,000 sources. You can access the full texts of about 60% of the indexed documents for free (Open Access). BASE is operated by Bielefeld University Library. 
OpenAIRE is a European project supporting Open Science. On the one hand OpenAIRE is an network of dedicated Open Science experts promoting and providing training on Open Science. On the other hand OpenAIRE is a technical infrastructure harvesting research output from connected data providers.
Dimensions is a dynamic, easy to use, linked-research data platform that re-imagines the way research can be discovered, accessed and analyzed. Within Dimensions, users can explore the connections between grants, publications, clinical trials, patents and policy documents. 
Handle.Net® - Thousands of handle services are currently running. Over 1000 of these are at universities and libraries, but they are also in operation at national laboratories, research groups, government agencies, and commercial enterprises, receiving over 200 million resolution requests per month. 
Index Copernicus - Index Copernicus (IC) is an online database of user-contributed information, including scientist profiles, as well as of scientific institutions, publications and projects established in 1999 in Poland, and operated by Index Copernicus International.*Wikipedia
WorldCat is a union catalog that itemizes the collections of 72,000 libraries in 170 countries and territories that participate in the Online Computer Library Center global cooperative. It is operated by OCLC Online Computer Library Center, Inc.*WorldCat (USEFUL online journal - print version)
*WorldCat (USEFUL online journal)
*WorldCat (DSpace at USEFUL.academy - repository)
Mendeley - Mendeley is a desktop and web program produced by Elsevier for managing and sharing research papers, discovering research data and collaborating online. It combines Mendeley Desktop, a PDF and reference management application available for Windows, macOS and Linux. *Wikipedia
ResearchBib - is an academic database that indexes and provides open access to peer-reviewed journals, full text papers, research conferences & positions.
OPEN ARCHIVES - is an organization to develop and apply technical interoperability standards for archives to share catalog information (metadata). *Wikipedia
ResearchGate - ResearchGate is the professional network for scientists and researchers. Over 15 million members from all over the world use it to share, discover, and discuss research. We're guided by our mission to connect the world of science and make research open to all.