Every summer, a new museum with neither roof nor walls arises on the National Mall. Its congeries of people, performances, lectures, processions, signs, and foods offer a somewhat incongruous presence on the nation's front lawn, flanked as it is by monuments, federal buildings, and national museums. But the Festival of American Folklife, now in its 29th year, has become a mainstay of the Smithsonian, an immensely popular exhibition of American and worldwide cultural heritage.