Rent Bits is a very accessible site where you can find information about available rentals all across the United States. You can search by city or by zip code. It seems to have one of the most complete, if not the most complete list of any other web tool out there.
This is another application that my students can use when they do their annual analysis of local neighborhoods, as well as a source of information when they study economics and have to develop a realistic idea of how it will cost them to live.
I’ve placed the link on my website under Student Neighborhood Maps.