The superb Musee D'Orsay by jrs1234
It's definitely worth your while spending time checking out some art at the excellent Musee D'Orsay. If your time is limited and you know you love Impressionist painting, go there first! The paintings are just amazing, and the building (a converted railway station) is cool too. This place is definitely somewhere I'll visit on every trip to Paris.A must for Van Gogh fans in particular - the collection of works here is small but first-class - and there are many other famous artists represented here as well.
A museum card is highly recommended if you're paying a visit - you get to skip the main queue for tickets. If you don't have one, adult entrance is 7 Euros, and don't forget it's closed on Monday. There's late opening on Thursday evenings.
For more information, check out the excellent museum website at www.musee-orsay.fr.