An interactive Pivot Table lets the user analyze a data set by selecting which fields to use as rows and columns in a grid. You've probably seen this in Excel, something like this:
This animated screenshot shows how the plugin is set up and used in Apex:
Note that you can have multiple Pivot tables on the same page if you want. You can also turn off the interactive features if you just want to display a preset summary.
You can download the plugin here. Enjoy!