While the weather shouldn't discourage you from visiting this beautiful region of Italy year-round, if you want to save money, enjoy a comfortable climate and avoid a throng of tourists, the best times to visit Tuscany are in the fall and winter months of the low season.
Winter is by far the least busy - it can get cold, but you can enjoy a wonderful trip here during this time of the year with uncrowded museums (no need to make reservations for the Uffizi and the Accademia galleries at this time of year!), and festive events such as markets and concerts around Christmas and New Years time. February and March are often cold and have shorter days, but it is a decidely quiet time of year, so expect to have museums and towns to yourselves.
In the spring and early summer months between April and June, the weather is usually ideal - it's not too hot, it's not too rainy, but it is more than ever becoming the most popular time of travel for many so do not expect to be the only person traveling through Tuscany at this time of year!
The summer months are the hottest and when the most tourists flock to the area, thereby driving up prices and decreasing availability for accommodation. June and July are some of the busiest months of the year - with temperatures high enough for beach-going and the fact that August is when most Europeans go on extended vacation, you'll find it hard to get affordable accommodations. In August, many businesses in Florence and other towns (particularly restaurants and family run businesses) close for a good part of the month while the locals have decided to take their own vacations - while anywhere along the coast livens up in the summer.
The hot summers in Florence are exacerbated by the city's location; surrounded by hills and far from the coast there is little in the way of summer breezes to freshen the sweltering city air. Many choose a day trip into Tuscany to escape the heat and tourist crowds during this time. Travellers planning to visit during the months of July, August and September would be well advised to check their hotel has air-conditioning: almost all do, but it is well worth checking in advance.
September is a busy month - but a wonderful time to visit weather-wise as it is usually cooler than August. Or if you like to wander around the towns and cities in the brisk fall with the colorful foliage, the autumn months will be ideal for your trip.The landscape is beautiful at this time and there are often many interesting festivals going on at this time of year as well, making it an ideal time to travel through Tuscany. Florence gets a bit quieter in the autumn and even if it rains, you can happily spend much more time in the less crowded museums and restaurants at a slow pace.