The three most famous museums in Paris are Le Louvre, Orsay and Beaubourg and all of them deserve it.
http://www.louvre.fr/
http://www.musee-orsay.fr/
http://www.cnac-gp.fr/
But, some other museums are worth the visit.
- Musée Guimet
> asian arts (top quality)
http://www.museeguimet.fr/
6 place d´Iéna - 75016 Paris
Phone: +33 1 56 52 53 00
Metro: Iena
- Musée Jacquemart-André
> French, Dutch and Italian paintings
http://www.musee-jacquemart-andre.com/
158 bd Haussmann - 75008 Paris
Phone: +33 1 45 62 11 59.
Metro: Saint-Augustin (line 9)
- Musée Carnavalet
> Museum about history of Paris; free entrance.
http://www.paris.fr/musees/musee_carnavalet/default.htm [French only]
23 rue de Sévigné - 75003 Paris
Phone: +33 1 44 59 58 58
Metro: Saint-Paul (line 1)