These names are familiar to anyone working with R, as all have contributed tremendously to the project. It's impossible to single out any one contribution, but here's my attempt. Duncan Murdoch is responsible for the Windows builds of R. Martyn Plummer contributed Bayesian and epidemiology packages. Martin Mächler maintains the Matrix package and many other numerical algorithms. Kurt Hornik maintains CRAN. John Chambers designed the object-orientation systems. Brian Ripley is a regular committer to the source code and mailing lists. Dirk Eddelbuettel maintains R on Debian.
The R Foundation has also recently added several new members to the leadership team (as "ordinary members"). Torsten Hothorn, Michael Lawrence, Uwe Ligges, Martin Morgan, Deepayan Sarkar, Marc Schwartz, Hadley Wickham and Achim Zeileis bring the total to 29. Former presidents Robert Gentleman and Ross Ihaka and former secretary Friedrich Leischremain on the board, as do auditors Peter Dalgaard and Roger Bivand.
I second Martin Mächler's sentiment from the announcement:
Thank you all for supporting the R project, a Free Software (hence Open source) endeavor only made possible by the dedicated work of many thousands of volunteers and their contributions in several ways, including to become supporting (or ++) members of the R foundation!
R-announce mailing list: The R Foundation - New board