The BASE search engine is designed to find academic open access resources. BASE is operated by the Bielefeld University Library. BASE provides an excellent 'Advanced Search' facility with options to limit by country, document type and publication year. Searches can be refined by content provider, subject and sorted by relevance. Records can also be checked on Google Scholar or directly exported to your referencing software.

BASE, OAIster and OpenDOAR are other examples of directories harvesting repositories using the Open Archives Initiative Protocol for Metadata Harvesting (OAI-PMH) protocol. The Registry of Open Access Repositories (ROAR) lists open access repositories around the world; it's interesting to search by country and by deposit activity.