My personal favorite:
http://www.wollersheim.com/index.html
I've also visited the three lakes winery (out in the middle of no where

)
http://www.fruitwine.com/
Three lakes is pretty good. They make a lot of cranberry wines, which are ok, but not really my cup of tea.
WI has quite a few wineries, but these are the two I've visited. Door county is a good place to visit in WI if you want to tour a few wineries. Pretty part of the state