Casa Lecaros is an ambient, post-colonial guest house with eight rooms. It's located about 4 blocks from the Plaza de Armas on a safe, well-lit street that is quiet at night (and seems to avoid the choking diesel fumes of major thoroughfares, such as Avenida del Sol).
We had an excellent stay from the moment we arrived. Our room on the second floor was sprawling and sunny, with a great view of both the street life, the mountains, and the moon at night -very romantic! Plenty of hot water and clean towels in the private bathrooms; the beds were cosy and warm, and the place was well-cleaned and maintained. Continental breakfast was served each morning in a sunny room - plenty of bread, fresh fruit, yogurt, juice, and cheese to get the day started right.
The highlight of our stay was the guests house's Cusquena owner, Nataly Lecaros, who speaks English. She greeted us with piping hot mugs of fresh mate de coca (tea made from coca leaves) to alleviate the altitude sickness. In fact, there was always a thermos of hot tea ready for us. My first several days in Cusco were for work, and she was happy to accommodate my colleague and I with plenty of quiet workspace. Although there's no internet at the house, there are plenty of internet cafes nearby, where 1 sol (about thirty cents) will get you an hour. (if you're emailing documents, be sure to bring an extra memory or two stick that you don't mind tossing, since it will very likely get a virus.)
My significant other joined me after a few days for the tourism part of our trip. Nataly spent ample time going over maps with us and giving us helpful suggestions about how to structure our visit. We were on a tight budget, and she had some great insider tips (which internet cafes worked, laundry places that would cut us a deal, how to make sure we didn't get ripped off by taxi drivers, etc., which places washed fruits and veggies in purified water, etc.) She held our luggage for us while we went to Machu Picchu and hiked the Sacred Valley. In addition to being a gracious host, she's happy to give you an inside view of life in Cusco. We spent a lot of time chatting with her about life in Cusco, Peruvian politics, etc.
Overall, we couldn't recommend the place more highly - it provides everything you need at a great price.