Zillow is an online real estate service. Instead of having to go visit houses in person, you can do so online, browsing by neighborhood and narrowing down what you're looking for based on a variety of parameters. It'll do some of the legwork for you by figuring out how your house stacks up against other houses in the area, including how much it's worth per square foot and how much other houses sold for in your area.
One of the neatest features, however, is the option to ask a question of the sellers or the real estate agents. You can do so without even having to register. In turn, the service has effectively created a very large directory of real estate agents whose properties can be browsed, including houses they've sold in the past.