The folks over at RStudio have been killing it lately. A few months ago they released their integration with knitr and RMarkdown. We use it here at tumblr for report generation and I absolutely love it.
Most recently, they released Shiny, which lets you build interactive visualizations in the browser with minimal effort. I haven’t played around with it yet but I’m really looking forward to doing so.
If you’ve built any interesting Shiny apps yet, leave a link in the comments. I’d love to see them.