Siena tourist information office
Piazza del Campo 56
tel. +39-0577-280-551
Terresiena.it
Also useful:
Museisenesi.org
Comune.siena.it
Sienaonline.it (private)
Sienafree.it (private)
Planning your time in Siena: Though you can see a few of its greatest hits in just half a day, Siena really deserves at least one good, full day.
In fact, Siena's riches of sights really make it hard to do in a single day, so you're better off spending the night here—that way, you can squeeze in some sightseeing on the first afternoon, and some more on the second morning.
Couple that fact with Siena's central location in Tuscany, and it makes a lot of sense for Siena to be one of the overnight stops on your tour of Tuscan hilltowns.