When in Siena, where to eat? 
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As a local and a kind of picky palate, I have to say that it’ll be difficult to find in town a gourmet lunch or dinner, able to remain unforgettable, at a good price and in a cosy spot. Tourists are everywhere and good good quality food is mostly just at home, on a Sunday, when Mom decides to prepare something special.
That said, here my suggestions:
Le Logge. Pricy but nice ambience, good ingredients, put together in a different, not obvious way. Light, small portions.
Tullio Tre Cristi for fish.
Regarding little trattorias… hard to find a real one, maybe Il Carroccio…maybe Osteria di via dei Rossi... try the Cinghiale al dolce e forte (wild-boar in a sweet-and-sour sauce). It’s the best in town and a very traditional dish.
Some other options are along Via Camollia.
 prosciutto
Good pizza: il Pomodorino for nice views  (close by the san Domenico church) and Tia Loca  but out of town.
If you want to have the best taste of local Prosciutto, a quick and very very tasty snack, stop at PRETTO, any time of the day. Good beer or a good glass of wine, at a good price, in a simple and pleasant ambience.
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Want to see where few locals stop? On a sunny day, the Piazza has lovely coffee bars serving simple food. At Bar il Palio you find salads, soups, snacks and bruschette.
If you want to see the hidden corner of some businessmen, Grattacielo is the place. Very basic and very authentic: a glass of wine and a selection of salads, prosciutto, cheese and veggies. Jummy, very cheap but not fancy!
If you are looking for a good cup of coffee Caffè Fiorella is a must. Very small and busy where almost all the locals go at least once a day. No chance to sit.
bisteccaRestaurants out of Siena
Just a few minutes driving from Siena town centre, la Taverna di Vagliagli, for a good T-bone steak and grilled meat.
La Bottega del 30, Villa a Sesta.
Il granaio in Rapolano Terme.
Lo Spugnone in Bagni San Filippo. Very traditional, very simple. A great familiar atmosphere, for a place out of the tourist traps, where to enjoy seasonal delis.
Silene in Seggiano, a little far from the most important and well known tourist stops, it’s a place that offers very good traditional Italian food with great truffle and mushroom dishes, Chianina beef and more.
Around Chianti, Solo Ciccia in Panzano is an interesting stop for those of you who love meat. The name of the place in fact is “just meat” and you’ll have it from the appetizer to the main course. Raw carpaccio, great meat sauce for pasta and meat meat meat!
cenaIf you are in Siena during the summer time, don’t forget to ask somebody how to reach one of the Contrada parties at night. In different weeks different neighbourhoods organizing the horse-race (Palio, July 2nd and August 16th), do celebrate their patron saints in a very unique way. They set in some of their gardens or streets, tables and chairs and they prepare various areas with sophisticated restaurants or very basic self-service options for dinner, beer corner, music, barbecue, and even a sort of local betting, through which you can buy a ticket representing on of the Contrada and if you win at the “barberi” (you need to experience it to know exactly what that means) you’ll get your price! A whole prosciutto, 12 bottles of wine or a cheese shape! It’s lots of fun, very good price and nice company. Dates may vary each year, so for farther info contact me, but for example the Nicchio Pania festival (my contrada) makes that for two weeks, in between the last one in July and the first one in August.