Airbnb is great- they have options to search for a whole place or to share a room for uber cheap etc. My sister had a relatively large bedroom in her house in San Fran with 2 beds and sometimes people would rent it out and sleep in the other bed - in a place like San Fran they would actually pay $50 or so to share a bedroom. We also rented a sweet apartment right in the Trastevere neighborhood in Rome earlier this summer from airbnb and it was great. Highly rec checking it out especially in places like Rome where you either are in a teeny guest room or very expensive hotel