RStan is the R interface to Stan.
RStan's home page, with links to everything you'll need to use RStan and Stan is:
RStan Getting Started
RStan's source-code repository is hosted here on GitHub. Stan's source repository is defined as a submodule here, see how to work with stan submodule in rstan repo.
RStan is licensed under GPLv3. The Stan code packaged in RStan is licensed under new BSD.