Repositories mostly fall into one of the following types:
The Registry of Open Access Repositories (ROAR) aims to promote the development of open access by providing information about the grown and status of repositories throughout the world.
OpenDOAR (the Open Directory of Open Access Repositories) creates an authoritative reference database of Open Access repositories worldwide.
Both of these two portals provide several filters including subject, repository type and content type (e.g. articles, theses, monographs, etc.).
OpenAIRE is an Open Access publications infrastructure, making visible funded research outcomes.
arXive.org is Open Access and has over 1.5 million e-prints in the fields of physics, mathematics, computer science, quantitative biology, quantitative finance, statistics, electrical engineering and systems science, and economics.
In other words, differing funders' requirements and publishers' licensing contracts will inform the choice of PubMed repository (i.e. any embargo on deposit of full texts may mean that the PubMed abstract remit is more appropriate than PMCentral).