The folks over at Rstudio have released a new update to their open-source R GUI, currently in beta test. This Beta 2 release adds more customizable layouts, editor improvements and new editing themes. Also of interest is a new feature that allows creation of graphics that update under the control of sliders and checkboxes and such. See the full details at the Rstudio blog, linked below.
Rstudio blog: RStudio Beta 2 (v0.93)