Found something on the internet and thought would be useful to folks here. We are travelling in Sep and will be sure to hit atleast one of these:
http://www.justfrance.org/france/paris/festivals.asp
In September:
« Fête de l'Humanité: Music, food and drink from different places and at a cheap price.
« Journées du Patrimoine: For two days people can visit important buildings like the Palais de l'Élysée, embassies and ministries that are not usually open to visitors.
« Techno Parade: A big parade from Place de la République to Pelouse de Reuilly
« Festival d'Automne: A theatre and musical festival with visitors and participants from America, Japan and Europe.