SF is the home of AirBnB. It's just been made "legal". Hosts are limited to 90 days per year (if the property is fully let out, no limit if part of a property is let out and the host lives-in) and need to register, pay the city's hotel tax.
http://techcrunch.com/2014/10/07/airbnb-sf/
http://techcrunch.com/2014/10/07/san-francisco-airbnb/
http://techcrunch.com/2014/10/07/airbnb-sf/
http://techcrunch.com/2014/10/07/san-francisco-airbnb/