Multiple times in the last 3-5 years, I have coded up wrappers to new/evolving C++ libraries using Rcpp for internal use by bioinformaticians.

Especially anything using multi-threaded algorithms or from the recent algorithms research world, I really get a kick out of hacking into a usable R module. It is almost like doing computer science again (as opposed to the real day job, which is data munging for reporting tools).