R is used to calculate statistics. The interface is not user friendly. But, once mastered, a lot can be achieved. Especially as it can be scripted.

Being open source, the system is continually extended, and efficient. It is also cross-platform. I use it regularly on both the Windows and Linux machines I work on.

The windows version is easy to install (download and run the .exe file) and includes a GUI:

The Linux version runs a terminal application. Though GUIs such as RCommander and RKWard are available.

Good tutorials are here (general), here (for Psychologists) and here (SPSS users).