Rserve package

Rserve is a package that can be installed with this command in R:

install.packages('Rserve')

Then you can start a R instance in server mode with the command:

R CMD Rserve

You can find more information about this package and its configuration on the project page. Several libraries gives the ability to communicate with this server through a TCP/IP interface. An interesting one is J2R developped in Java.