gWidgets
The gWidgets package for the R statistical computing environment provides a lightweight means to write GUIs for R within R. There are several R packages that provide access to underlying GUI toolkits on a system, such as tcltk, RGtk2, rJava and RwxWidgets. The gWidgets packages provides an interface to these packages that is toolkit independent. Although not nearly as feature rich as any of the underlying toolkits, gWidgets provides an API that allows many things to be done quite simply and in a more or less portable manner.
There are a number of examples in the Examples directory, and the package has an accompanying vignette.
11/17/2008: New versions of gWidgets, gWidgetsRGtk2, gWidgetstcltk, gWidgetsrJava
A few additions (gaction, tooltip), and a lot of bug fixes.