Click on him, and his peg leg drops in a fountain of gold.
In the west corner of Tristram in Act I, Wirt's body can be found lying by the river, with a Carver Shaman and Carvers always nearby. However, you need to find some items before you can do so. After completing a difficulty by killing Diablo (or Baal in Diablo II: Lord of Destruction), you can open a portal to the fabled Secret Cow Level back in the Rogue Encampment of Act I.
