Alma Domus is a modest hostel run by nuns of the same order of Siena's famous St. Catherine. My first visit to the hostel was by chance, when we arrived in Siena without reservations and happened upon Alma Domus after finding no vacancies elsewhere. Welcomed in by a pleasant Sister who showed us a clean room (with crucifix) over the bed, and bade us goodnight, we awoke to the sound of church bells, swallows, and a spectacular view of Siena and 'The Cathedral' from our balcony.
The hostel is strategically located next to the Basilica San Domenico, which houses a fresco of St. Catherine, as well as a few fascinating reliquaries (including her finger!) Our double room with bath had ample space and was well tended. The view makes the room seem gourmet! My single room on my second visit was immaculate, though the bed was a little lumpy. I was impressed by the cleanliness of the share bath on the second visit as well. Having the nuns at the door gives you a strange sense of security, even if you're not religious.
The only drawback to Alma Domus is the curfew, which was 11:30pm -- a late dinner with lots of wine can send you scurrying through the windy streets to make it home in time for the Sisters to let you in! I'd also highly recommend making reservations in advance of your arrival, at least a week or two -- during my last visit to Italy I desperately wanted to stay there, and was met by disappointment when I called a few days ahead to find they were booked!