One of the most impressive churches in Paris - La Madeleine - offers regular Sunday concerts most of which are with free admission. Occasionally concerts are held during other weekdays as well. Check timetable and program here.
Autumn series of free baroque music concerts in Marseilles include eight concerts at Marseille's Saint Michel church with Haydn, Bach and Mozart - traditionally in October-November. Timetable check at http://www.marseille.fr/siteculture.
Since 2002 the Tout l'monde dehors or 'Everybody Outside' festival has been bringing open air arts and music to Lyon, completely free of charge, throughout the summer from mid June to September. Check program here.
These shows are operas composed of sounds and images and are projected on a transparent tulle screen measuring over 100 square meters hanging in the nave.
Notre-Dame de Paris is a church, a place of Roman Catholic worship, where Christians come together to pray. A Gothic masterpiece on the UNESCO World Heritage List that was celebrated by Victor Hugo's novel published in 1831. Access to the cathedral is open and free of charge every day of the year, during the opening hours.