While the Louvre (above) may be more famous than the Musee d'Orsay (below), I prefer the latter. The d'Orsay building was originally an old train station (super cool!) and today has a lovely collection of art from the late 19th and early 20th centuries. My favorite works? Henri de Toulouse-Lautrec's charcoal masterpieces.
I'm hoping to go to more museums this summer in the San Francisco area: I definitely want to see the new King Tut exhibit at the de Young. Any other suggestions? What are your favorite museums?