The Wisconsin Geological and Natural History Survey has a well organized, appealing website. Their data is easy to find although there is not much GIS data available. Although because I am more interested in sample data sets than anything, the GIS data they do have–statewide geologic map; pleistocene geology, precambrian geology, quaternary geology, (surficial?) geology and bedrock geology for various counties–may actually be a great reference. Other than the statewide geologic map, the data is available on a per-publication basis.