Craigslist has RSS feeds that respect categories/search terms: http://www.craigslist.org/about/rss :) You're technically not supposed to use them for search engines specific to housing, but Craigslist doesn't seem very serious about enforcement.
I wonder if landlords will go after this like the hotel industry went after AirBnB. Any accurate information a potential renter has about other apartments in the area can only hurt the landlord.