Hefewiezen are pretty near and dear to my heart.
Franziskaner, Paulaner, and Hacker-Pschorr are German imports that are pretty typical of the style. They are widely available here in the US. Hacker-Pschorr is my sentimental favorite of the bunch, mainly because it is the first wheat beer I ever had.
I haven't found a domestic beer that has the same complexity of flavors as the German imports, but Summit makes an ok wheat beer. Blue Moon is a unfiltered wheat ale from Coors, of all people, that gets tremendously wide distribution and isn't terrible for a mass-market commodity beer.
Anybody reading this thread who finds themselves in Atlanta must go to Five Seasons Brewing Company:
http://www.5seasonsbrewing.com/
They are the one and only contender for best brewpub in the Atlanta area. They make a Big Blonde Hefewiezen that is absolutley out of this world. They brew seasonally, only keeping four or five styles on the menu at a time, and I've never had a beer there I didn't like. Their food is really great, too.