This is the first of a few showdowns, or throwdowns if you prefer, comparing open source GIS' spatial analysis tools to ArcGIS. This week: Thiessen polygons. You will need an ArcGIS Advanced Desktop (formerly ArcInfo) license to create these, or some patience with open source software.
See below for a comparison. Unfortunately, QGIS produced some different/strange results. I'm not sure why this is but I am investigating. Haven't tried with pysal yet. Anyway, see below. Fyi.