MUSEUMS:

This is a link to a list of the temporary exhibitions going on in Paris. It is also a site covering museums all over France Réunion des musées nationaux

Following are some of the museums that are free the first Sunday of the month (only their permanent collections however):
- Louvre
- Centre Pompidou

Lots of museums have reduced prices on Sundays and if you are a student or teacher or under 26 or above 60 (or all of the above) then you get a good price as well. (Remember some ID)

If you're under 18 you get in for free most places.

And these are some of the museums that are always free (again, permanent collection only)
- Musée Carnavalet
- Maison de Balzac
- Musée d'Art moderne moderne de la Ville de Paris

Museum passes:
La Carte Musées & Monuments are sold for 1/3/5 days at paticipating venues, FNAC and tourist offices. You can also buy them here

Tarifs à titre indicatif:
Carte 1 jour: 15 €
Carte 3 jours: 30 €
Carte 5 jours: 45 €

Late night museums:
Louvre is open till 9:45 PM on Monday and Wednesday. To avoid the queue outside the pyramid go through the Carrousel du Louvre (Rue de Rivoli)

Musée d'Orsay is open till 9:45 PM on Thursdays.

Full listing:
Paris Touristoffice

Art supplies:
If you're into making your own creations then Graphigro has a good selection of various art supplies at a fair price.

133 r Rennes (6 arr)
207 bd Voltaire (11 arr)
157 r Lecourbe (15 arr)
120 r Damrémont (18 arr)