R comes with a reasonable interface (or integrated development environment, IDE, which in turn is a graphical user interface, or GUI). Many people are happy using it as is. I use R with Emacs and ESS and almost always write code using the Sweave markup system for combining R and LaTeX code. There are many other options:

• Wikipedia list of interfaces for R
• R wiki list of guis
• R wiki list of IDEs which are also GUIs
• An Eclipse plugin for R

I have really liked the look and feel of RStudio . And my undergraduate and graduate students have been using it for their own work in R. If you like VI, there are also plugins available for you, too.