From the central station of Florence you can easily reach most places in Tuscany, including:
Siena 1.5 to 2 hours
Pisa 1 to 1.5 hours
San Gimignano by train to Poggibonsi, 1 hour ride, and then a bus that runs every 30-40 minutes, 25 minute ride
Volterra also reachable by bus from Poggibonsi
Toscana Mobilitá has a useful website (http://www.busfox.com/tim...) for bus routes and schedules in Tuscany. The site is mostly in Italian, but is simple to use. The Tuscan bus companies Siena Mobilitá, Tiemme and Toscana Mobilitá seem to be affiliated.
The website for Siena Mobilità (http://sienamobilita.it/o...) has bus schedules orari for and between a number of popular towns in Tuscany including Florence Firenze in the schedule, Siena, San Gimignano, Arezzo, Cortona, Montepulciano and Chiusi among others. Local services for several cities are marked urbano. The interurban services are all under the tab servizio extraurbano.
Google maps identify bus stops throughout Tuscany for both local and interurban routes. If you click on the bus stop symbol, you can get a list of bus routes serving that stop. Using Google Streetview, you can often identify which side of the road the stop is situated and hence which direction of travel is served by the stop.
Bus users should purchase their bus tickets before boarding the bus. Most Tabacchi-shops tobacconists sell bus tickets. Sometimes newsstands and bars may also sell tickets. You must tell the ticket seller your destination so that your ticket will be valid for the correct fare zones. After boarding the bus, stamp your ticket in the machine located behind the driver.
Be aware that many routes have either reduced or no service on Saturdays, Sundays and holidays. Schedules indicate reduced service as festivo while the regular work day schedule is feriale. Many bus stops have posted schedules.
Blue-coloured buses are for interurban service while orange-coloured buses are for local service. Interurban buses can serve local stops along the route.