darwin8 = for Mac OS X 10.4 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 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 (32-bit only, Intel and PPC)
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.