R -Fiddle is a free and powerful tool to write, run and share R-code inside your browser. Include packages to really run R in the cloud.
DataMind is the first free online Interactive Learning Platform for R. Through an in-browser coding environment you can learn R by doing.
Search R documentation and manuals instantly with the help of RDocumentation.
This post summarizes and discusses the results of DataCamp's survey on “R and education”.