Via the OGD blog I learned about OpenGeoDa, an open source software program that serves as an introduction to spatial data analysis. The initial (closed) version was released in 2003 and they claim about 70,000 users. While we mentioned GeoDa before, it's really the first time we share about its existence with our users.
Here's what it is: "It is designed to implement techniques for exploratory spatial data analysis (ESDA) on lattice data (points and polygons). The free program provides a user friendly and graphical interface to methods of descriptive spatial data analysis, such as spatial autocorrelation statistics, as well as basic spatial regression functionality. The latest version contains several new features such as a cartogram, a refined map movie, parallel coordinate plot, 3D visualization, conditional plots (and maps) and spatial regression."