R is a statistical computing language that runs on many different platforms (Windows, Mac, and Unix/Linux included).
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Pro Lots of packages available
There are lots of different packages available that can be easily searched from the CRAN repo site and downloaded, or installed via the R command line interface. These packages are easy to include in a project or source file, and pertain to a wide variety of topics, from classification, social media analysis, and text processing to interactive 3d plotting and networks (including neural nets).
Pro Has a wide range of options when it comes to IDEs/GUIs
Among the IDEs available there are several commercial applications as well as free and/or open-source ones, such as R Studio, which features syntax highlighting, project management capabilities, integrated terminal access, decent code completion and on-the-spot parameter hinting, graphical interfaces for package installation and such, and commendable extensibility/developer support.