R Project for Statistical Computing
The R Project for Statistical Computing is used heavily in the research and academic communities for data analysis, and it is also well suited for visualization. As with QGIS, there are a number of plug-ins as well as tools that extend its capabilities considerably.
Basic R is a free command-line tool that runs on Windows, Mac and various Unix platforms, but there's a whole ecosystem of tools around the platform as well. For example, RStudio is a free IDE designed for use with R. There's even a plug-in (free for non-commercial use only) for use within Excel.
Get it: http://www.r-project.org