The smaller and newer of the two unions at the University of Wisconsin-Madison. It was built in the 1970s, but in spite of this, actually looks pretty decent. Not many organizations have offices there, but there are quite a few meeting rooms. Union South is home to the Red Cross's on-campus blood donation center, Young Blood. There is also a bowling alley in the basement.
Union South is very close to the engineering and computer science buildings, and as such, is very popular among those majors. I myself have spent far more on coffee there than I wish to recall. Good coffee, though.