One of the best hotel booking services for last-minute reservations (up to 21 days in advance) I have found so far is Rates to go.There are some really good deals and I have not been disappointed by the more than half a dozen hotels I have booked through that service in France, Italy, Czech Republic and Spain.
However, I would always recommend doing some comparison shopping because although some of the offers are really good, others are actually quite the opposite. For comparison shopping, Expedia or Hotelreservations work well, which are also superior when it comes to booking hotels outside Europe.
For Italy, the largest selection is offered by Venere. (Deals for other countries are not always that great.)