Many institutions have created digital repositories to store a wide range of scholarly material including research papers, student theses, scientific data. The drivers behind this are to make research material available as widely as possible, so authors are encouraged to self-archive as well as publish in peer-reviewed journals. For a broader discussion of issues around creating e-print repositories see this paper Creating institutional e-print repositories by Stephen Pinfield, Co-Director of the SHERPA project which has established several e-print repositories in UK universities.
Here are links to some of the major search services for a range of repositories:
Sherpa UK (coverage: UK, all disciplines)
EThOS – (coverage: UK, all disciplines, theses)
BASE (coverage: global, all disciplines)
CORE (coverage: global, all disciplines)
OAIster (coverage: global, all disciplines)
OpenDOAR (coverage: global, all disciplines)
SORA (St George’s Online Research Archive)
There are also several search services to find repositories including:
Registry of Open Access Repositories (ROAR)
Updated: December 12 2016 to remove redundant links.