With the change in policy from HEFCE in relation to the REF, it looks like there is some impetus for these long-standing initiatives below to come into their own. Most useful in this list, which I collated a couple of years ago, are CORE, DOAB, DOAJ, EThOS, OAIster, and PQDT Open.
CORE - an aggregated search of UK repositories (it's like Google Scholar for UK research repositories). Further information.
DART-Europe - research theses from a consortium of European research libraries
DOAB - Directory of Open Access Books
DOAJ - Directory of Open Access Journals
EThOS [Electronic Theses Online Service from the British Library. Selected items available for download after registration]
OAPEN [Open Access Publishing in European Networks] is a collaborative
initiative to develop and implement a sustainable Open Access publication model
for academic books in the Humanities and Social Sciences.
OAIster - a union catalogue of open access resources, provided by OCLC/Worldcat. Further information
OpenDOAR Search - search contents of open access repositories listed on OpenDOAR [Directory of Open Access Repositories]
OpenDepot Search - search of e-prints from researchers at institutions that do not have an institutional repository
PQDT Open [Proquest Dissertations and Theses Open] - provides the full text of open access dissertations and theses available on the Proquest platform
ROAR Search - search contents of repositories listed on ROAR [Registry of Open Access Repositories]
SHERPA Search - search UK repositories listed on OpenDOAR [Directory of Open Access Repositories]