I have been consulting for this website, Tuscany Rent Direct, that specializes in direct to consumer rentals in Tuscany, including Florence, Siena, Chianti wine country and beyond. They have a nice and ever expanding selection of apartments in Florence, and I think after a few months, we have made some positive changes to the user experience of the site. If you get a chance, check it out - and if you or anyone you know is looking for a rental villa or apartment anywhere in Tuscany, please pass this along!
Also - if you own a villa or apartment in the region and have it listed for rent or are considering it, check out their options for owners. There is a 6 month free listing offer available and you get a lot - photos, floor plans, Google map locations, SMS alerts, etc.