rdatacite is an R package that provides programmatic accesses to DataCite's metadata using their OAI-PMH harvester. The package does not require an API key. Documentation for the DataCite OAI-PMH is available here.
install.packages("devtools") require(devtools) install_github("rdatacite", "ropensci") require(rdatacite)
rdatacite is part of the rOpenSci project, visit http://ropensci.org to learn more.