From the website:
RStudio™ is a new integrated development environment (IDE) for R. RStudio combines an intuitive user interface with powerful coding tools to help you get the most out of R.
RStudio brings together everything you need to be productive with R in a single, customizable environment. Its intuitive interface and powerful coding tools help you get work done faster.
RStudio is available for all major platforms including Windows, Mac OS X, and Linux. It can even run alongside R on a server, enabling multiple users to access the RStudio IDE using a web browser.
Free & Open
Like R, RStudio is available under an open source license that guarantees the freedom to share and change the software, and to make sure it remains free software for all its users.
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