While Paris does have some great free museums and art galleries, such as Le Petit Palais and Musée d'Art Moderne de la ville de Paris many of the museums have an entrance fee. Since most of my museum going "at home" is in Oxford or London, where the bulk of the museums are free, it's a little shock to be asked for €10-€15 to see the permanent collection.
To help avoid this, are there any special days / times / dates when many of the Parisian museums and galleries are open for free, instead of their usual entrance fees?