Osteria del Gatto
Via San Marco 8
near porta San Marco - Siena
This is a small restaurant that serves typical Sienese dishes. Your host and cook Marco will happily recommend the best items on the menu and serve them to you with an excellent wine from his cellar. On Fridays there is a menu of fish cooked in a Tuscan way.
Not only is the food excellent but the price is reasonable.
Tel: +39 0577 287133

Antica Trattoria Papei
Piazza del Mercato, 6
A Michelin recommendation.
Closed Monday
Tel: +39 0577 280894

Al Mangia
Piazza del Campo
The best (and most expensive) on the Campo
Tel: +39 0577 281121

Il Ghibellino
At the Baptistery end of via dei Pellegrini 
An attractive traditional atmosphere, excellent reasonably priced country food, Italian clientele - the preferred place for a midday stop between Duomo and Campo!
Closed Monday
Tel: +39 0577 288079