Cross Repository Search Engines

• OAIster A union catalog of millions of records representing open access resources that was built by harvesting from open access collections worldwide using the Open Archives Initiative Protocol for Metadata Harvesting (OAI-PMH). Today, OAIster includes more than 25 million records representing digital resources from more than 1,100 contributors. Includes articles, images, audio, maps, scores, and many other formats. 
• BASE BASE is one of the world's most voluminous search engines especially for academic open access web resources. BASE is operated by Bielefeld University Library. BASE provides more than 36 million documents from more than 2,200 sources. You can access the full texts of about 75% of the indexed documents.
• OpenDOAR   Aims to"provide a comprehensive and authoritative list of institutional and subject-based repositories, as well as archives set up by funding agencies." Search both for and across repositories.
• Bepress  Provides digital publishing solutions for institutions as well as a searchable collection of those institutions' content.
• Europeana offers "millions of books, paintings, Europeana films, museum objects and archival records that have been digitised throughout Europe. It is an authoritative source of information coming from European cultural and scientific institutions."

Broadly Relevant Repostories

• SSRN Social Science Research Network (SSRN) is devoted to the rapid worldwide dissemination of social science research and is composed of a number of specialized research networks in each of the social sciences. Pre-print articles and other submissions are not necessarily peer reviewed.
• World Bank Open Knowledge Repository The World Bank is the largest single source of development knowledge. The World Bank Open Knowledge Repository (OKR) is The World Bank’s official open access repository for its research outputs and knowledge products.Through the OKR, The World Bank collects, disseminates, and permanently preserves its intellectual output in digital form. 
• World Digital Library The World Digital Library (WDL) makes available on the Internet, free of charge and in multilingual format, significant primary materials from countries and cultures around the world.
• National Library of Australia Trove is the repository of the National Library of Australia. Limit search to "freely available" for an OA search of millions of items, although much overlaps with other sources on this page.

• Directories—E-Prints, Institutional Repositories, and Technical Reports Although there's no search capability, this is a fine webliography of OA repositories. 
• Registry of Open Access Repositories (ROAR) Seeks to provide "timely information about the growth and status of repositories throughout the world." Includes visualizations of repository size and growth.

Central Asia Specific Collections