DOAJ is an online directory that indexes and provides access to high quality, open access, peer-reviewed journals.
The primary aim of DOAB is to increase discoverability of Open Access books. It supports the OAI protocol for metadata harvesting.
The Open Access Directory (OAD) is a compendium of simple factual lists about open access (OA) to science and scholarship.
OpenDOAR is an authoritative directory of academic open access repositories.