RcppArmadillo, RcppEigen and RcppGSL extend, respectively, to the Armadillo, Eigen and GSL frameworks. These packages are authored jointly by the Rcpp Core team.
A few smaller and experimental packages use the facilities provided by Rcpp to extend to other interesting package or libraries. A few examples are RcppBDT, RcppCNPy, RcppSMC, RcppXts.
Another key package is RInside which uses Rcpp for data transfer when embedding R in a very easy-to-use C++ class for deployment in other programs.
digest provides hash sums of R objects, and is used by a number of R packages providing various cacheing solutions.
BH provides Boost Header for use by R.
RPostgresql connects R to Postgres. This is now maintained Tomoaki Nishiyama.
Other projects include the old-yet-still-used Perl-based beancounter (and smtm) as well as Finance::YahooQuote.