Not cheap but useful!

While you are walking through the city you can dial : 0892 35 00 94 (0,34€ per minutes) from your cell phone and then you type a special code and You will have a three-minute commentary on the monument that you are in front of.

List codes for english commentaries :

300 : Chateau de Versailles
303 : Arc de Triomphe
311 : Centre Pompidou (Beaubourg)
335 : The Eiffel Tower
355 : Musée du Louvre
368 : Notre Dame
371 : Musée d’Orsay
396 : le Sacré Coeur