One can see art nouveau at Paris metro stations, some of which have curved railings and signs. Art nouveau was “was inspired by natural forms and structures, not only in flowers and plants, but also in curved lines. Architects tried to harmonize with the natural environment”.
In Paris we dined at the Bouillon Racine, a restaurant built in 1906 with an art nouveau style. The upstairs dining room has curved light fixtures, chairs, and mirror moldings. (Click on photos to see enlargements.)
Their metal and glass lamps are also art nouveau.
Art Nouveau architecture made use of many technological innovations of the late 19th century, especially the use of exposed iron and large, irregularly shaped pieces of glass for architecture. By the start of World War I, however, the stylised nature of Art Nouveau design—which was expensive to produce—began to be disused in favour of more streamlined, rectilinear modernism, which was cheaper and thought to be more faithful to the plainer industrial aesthetic that became Art Deco.
See my New York art deco post for examples.