darwin8 = for Mac OS X 10.4 (Tiger) or higher
darwin89= 32-bit for Mac OS X 10.4 or higher, 64-bit for Mac OS X 10.5 or higher
darwin9 = for Mac OS X 10.5 (Leopard) or higher
darwin10 = for Mac OS X 10.6 (Snow Leopard) or higher
darwin13 = for Mac OS X 10.9 (Mavericks) or higher
darwin15 = for Mac OS X 10.11 (El Capitan) or higher
It is safe to use darwin8 binaries on Mac OX S 10.5 (Leopard). Since R 2.10.1 we do not release darwin8 binaries anymore.
bin4 = quad-arch binary (32-bit and 64-bit, Intel and PPC)
bin3 = tri-arch binary (32-bit PowerPC and Intel, 64-bit Intel)
bin2 = universal binary (darwin8/9: 32-bit only, Intel and PPC; darwin10/11: Intel 32bit + 64-bit)
x86_64 = single-arch 64-bit, Intel (omitted for darwin13 binaries which are always x86_64)
Binaries built automatically via recipes are tagged with <system>.<version>-<arch>
Install by unpacking to the root, e.g.:
curl -O http://r.research.att.com/libs/gmp-4.2.4-darwin8-bin4.tar.gz sudo tar fvxz gmp-4.2.4-darwin8-bin4.tar.gz -C /Most binaries (except for GTK+) live in /usr/local and are static PIC builds (i.e. they can be used in R packages without the need for runtime dylibs).
For more libraries, see the CRAN devpack. Sources for most of the above are available at the project sites or here.