BASE indexed 50 million OAI-records by Sarah Dister.
From the post:
BASE, a search engine for academic open access web resources, has indexed more than 50,000,000 OAI-records. The records are provided by about 2,700 repositories among which many are related to agriculture.
BASE is a multi-disciplinary search engine for academically relevant OAI-Sources worldwide, which was created and developed by Bielefeld University Library.
Take a few minutes (or longer) to explore BASE.
It is a remarkable resource. For example, users can invoke the Eurovoc Thesaurus as part of their search query.