The Public Libraries of Science (PLOS) has an API so that developers can create cool tools to access their data (including full text papers!!).
Carl Boettiger at UC Davis and I are working on R functions that use the PLoS API. See our code on Github here. See the wiki at the Github page for examples of use. We hope to deploy rplos as a package someday soon. Please feel free to suggest changes/additions rplos in the comments below or on the Github/rplos site.
Get your own API key here.