Musee d'Art Moderne de Paris fills the eastern wing of the Palais de Tokyo with the city's modern and contemporary art collection. The holdings run from the early 20th century onward, with paintings, sculpture, photography, and large decorative works that give Paris another major art stop beyond the Louvre-Orsay-Pompidou circuit.
It is especially useful if you are already near Trocadero or the Eiffel Tower and want a museum with serious depth but less pressure than the city's headline institutions. The building and Seine-side setting also make it easy to fold into a wider walk through western Paris.