Zillow and Trulia allow users to search homes for sale in particular zip codes in addition to providing estimates about the value of property. Searchers interested in lots can find information on these sites as well. However, it's important to corroborate information to be sure it's accurate.
Both Trulia and Zillow combine information from a variety of sources including public records and information found online. They create algorithms for estimates about the value of property and homes in particular areas. While these estimates are generally fairly reliable, they can't take all factors into account; a home in poor condition, for example, is worth less than one that's been maintained well. Still, they serve as fairly accurate guidelines for expected prices within zip codes and even smaller zones.
However, Zillow and Trulia guard their proprietary algorithms used to determine the value of real estate, and showing the value of land excluding buildings on it would reveal more information than they're willing to release. As a result, they don't share the value of land in particular areas unless it's listed as an empty lot. Because of this, shoppers looking to invest in land likely want to consult with local real estate experts who can also share information on zoning and other factors than can affect the value of land.