There are two things we always looked forward to in September - the Festa della Rificolona parade, and the wine events of the Chianti Classico region.
The Festa della Rificolona (more photos) celebrates the Feast of The Madonna but it is much more than that - it is something steeped in mystery and tradition, and somehow remains a vibrant and magical evening. If you are in Florence on September 7th do not miss it - you can find people milling around near dark (around 9 PM) in either Piazza Santa Croce or Piazza Signoria, and a band of some sort than leads a procession through Florence, around the Baptistry, and up via dei Servi to Piazza Santissima Annunziata.
As to wine events - they are plentiful and memorable. There are two main events/tastings - the "Expo del Chianti Classico" in Greve in Chianti (September 11th to 14th), and, my personal favorite - Vino al Vino in Panzano (September 18th to 21st). Both can be absolutely over run and seem to be more popular every year, so plan ahead!
There are of course dozens of other events and things to do - we'll be updating the calendar soon with more dates!