If you are interested in Photo art than Le maison européenne de la Photographie is the place to be. They have notable shows and great retrospectives.
Until Aug. 2009 there is a interesting exhibition of Henri Cartier-Bresson. They do expose 320 works of the own collection. It has been a long work with a nice result.
Maison Européenne de la Photographie, 5-7, Rue de Fourcy, 75004 Paris, M°Saint-Paul