It's rare for the price of an add-in to approach that of the program it supports, but that's the case with XLStat. The saving grace, in this instance, is that you get some tools that are genuinely useful. Even the more elementary features of this program are well thought out: You can flag data within an Excel table, for instance, by changing the color or the size of data to fit a criterion you define.
You also can create box plots and stem-leaf plots to better visualize your data or compare several variables. The program's categorical sorting allows you to sort by more than one variable (it will first sort by the first variable, then the second, and so on).
This add-in's advanced features let you perform cluster analysis, multidimensional scaling, and nonlinear regressions.