HotPads shows real estate rents, foreclosure, demographic heat maps
Want to know how crowded the area is? Just click the population density heat map option and you'll get a good idea of how many neighbors will be sucking up your oxygen. You can also apply heat maps for per capita income, household income, median age, and median rent. There are even heat maps that show you the foreclosure rate per household, and the rent ratio. What's rent ratio? It's a comparison of rental rates versus mortgage rates in an area. In other words, if you see lots of blue, it means you'll probably save money by purchasing a home, while if an area is red, you're probably better off renting.
If you're looking for more mundane information, you can also plot schools, universities, and subway stops on the map.