The Dirndl is the traditional dress for Bavarian ladies. You can see waitresses wearing Dirndls at any of the Bavarian restaurants and beer gardens in Munich. At the Oktoberfest pretty much all girls are wearing Dirndls. Even the non-Bavarians and non-Germans.
For information on where to buy a Dirndl in Munich, see: Dirndl shops
Note that the equivalent attire for men is Lederhosen.
Together Lederhosen and Dirndls are known as Tracht.
Dirndls are hot. Fact!