From the post:
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.
As the open access movement grows and prospers, more and more repository servers come into being which use the “Open Archives Initiative Protocol for Metadata Harvesting” (OAI-PMH) for providing their contents. BASE collects, normalises, and indexes these data. BASE provides more than 70 million documents from more than 3,000 sources. You can access the full texts of about 70% of the indexed documents. The index is continuously enhanced by integrating further OAI sources as well as local sources. Our OAI-PMH Blog communicates information related to harvesting and aggregating activities performed for BASE.
One feature of the search interface that is particularly appealing is the ability to “boost” open access documents, request verbatim search, request additional word forms, and to invoke multilingual synonyms (Eurovoc Thesaurus).
I first saw this in a tweet by Amanda French