- [Microsoft R Open](https://mran.microsoft.com/open), formerly known as
  Revolution R Open (RRO), is the enhanced distribution of R from Microsoft.

- [Rho](https://github.com/rho-devel/rho) (previously called CXXR, short for
  C++), a re-implementation of the R interpreter for speed and efficiency.

- [pqrR](http://www.pqr-project.org/) (pretty quick R) is a new version of the R
  interpreter. One major downside, is that it is based on R-2.15.0.

- [Renjin](http://www.renjin.org/) reimplements the R interpreter in Java, so it
  can run on the Java Virtual Machine (JVM).
