If you download R or Revolution R Open, the R interface is pretty stark — you'll get a command prompt, and not much else. That's fine for quick, interactive calculations, but if you need to do any serious scripting or programming in R, you'll need an interactive development environment (IDE) to be productive. For subscribers to Revolution R Enterprise on Windows there's the DevelopR IDE, and for everyone else there's RStudio: the open-source, cross-platform IDE for R which was upgraded just this week.
RStudio provides a smart editor for the R language with syntax highlighting, code completion, smart indentation and interactive debugging. RStudio also makes it easy to work with disctinct projects in R, and to switch between them with ease. The new version 0.99 also adds a powerful data viewer with sorting and filtering, and enhanced R code completion functionality (which saves time and prevents errors if you write a lot of R code). RStudio 0.99 is available for free download for Windows, Mac and Linux systems, and it works with all recent versions of R and Revolution R Open.
RStudio blog: New Version of RStudio (v0.99) Available Now