Recently I was working on a project which requires analysis of thousands of traffic data. Unfortunately, I have to use Excel since I don't have access to software like SPSS or SAP. Excel gave me a headache because it runs slowly and you cannot easily generate some special graphs and change the settings.
A friend recommended
R -a language and environment for
statistical computing and graphics developed at Bell Laboratories.
R provides a wide variety of statistical (linear and nonlinear
modelling, classical statistical tests, time-series analysis,
classification, clustering, ...) and can produce well-designed plots. Users have full control (I like it!).
Most of all, it is Free Software :)