In the course of my research projects, I have found it useful to develop a few R packages, which can all be found on my GitHub. A few are in mature stages of development:
RcppDist provides header-only C++ library with functions for additional statistical distributions that can be called from C++ when writing code using Rcpp or RcppArmadillo. I had occasion to draw up the C++ code for these distributions on various projects, and rolled them all together into an R package and got it up on CRAN so no one else would have to.
bggum provides R tools for a Bayesian approach to estimating Generalized Graded Unfolding Model (GGUM) (Roberts, Donoghue, and Laughlin 2000) parameters. See the description of “Ends Against the Middle: Scaling Votes when Ideological Opposites Behave the Same for Antithetical Reasons,” my paper with Jacob Montgomery, on my research page for more information.