There are a lot of sites to scan if you're looking for real estate (a house to buy, or a rental), but one of the most useful, I think, is HotPads. The site was started as a rentals locator, but it has recently expanded to include homes for sale as well.
Like other sites, HotPads displays properties on a zoomable and movable map. But where HotPads shines is its display of additional data. It will overlay heat maps showing data like household income and average rental price, and pop schools and transit stops on to the map as well. However, while the schools database is very useful, I found the transit data incomplete - in San Francisco it shows streetcar stops only, but no info on buses.
HotPads gets its data directly from large rental management and real estate companies, which it has now tapped to provide it with home sales info as well. Individuals can enter listings, too. I looked at HotPads results for some San Francisco neighborhoods and found an impressive number of listings, and significant but not complete overlap with Zillow and Trulia. All services show houses for sale that the others do not.
Trulia also has neighborhood heat map data. See also GeoCommons.