I’ve been using Eclipse/StatET. However, there’s a new FLOSS IDE that was released today that looks interesting called RStudio. It even supports Sweave, so once it becomes more polished I may switch. User input seemed a little sluggish when I played with it earlier today however.
Update
They have been actively updating RStudio every month or two. It’s been months since I encountered even a minor bug. And the features added are really useful–seamless package development and native Git support being the two that come to mind.