Because it is part of the MLS, the site doesn't disclose exact addresses if the seller prefers not to and does not display automated valuation data if the listing agent doesn't want it shown. In some cities it serves, Redfin provides a complete sales and listing history. Otherwise, you can usually find that information on other websites.
Redfin's listings come from the MLS, which means they are up to date. Its real-time alerts are often the first to arrive, usually within 15 minutes of a listing going into the MLS.
Redfin, like several of the services, uses an algorithm to show you homes you might be interested in based on your searches and the homes you have viewed. You also can search for homes by listing the school you want your kids to attend.
Zillow rose to fame with its Zestimates, home value estimates derived from a proprietary formula that includes public records of home sales. As Zillow has gained more data, the estimates have gotten closer to reality, though they are still estimates.
Information from old listings lives on at Zillow, which can be valuable if a new listing doesn't include photos. It also has data on homes that are not for sale, and you can click on the properties near those you're looking at to see sales history, old listing data and other information.
Zillow has the most complete transaction history, which is helpful when you're trying to find out how long a home has really been on the market. It also includes data on homes that are in preforeclosure status and satellite maps that let you see what's near the homes you're looking at.
The service pulls data from a variety of sources, including public records, real estate agents, homeowners, tax assessors and some MLS services. Buyers and sellers can go online and update Zillow's data on their homes or list homes for sale or for rent.
However, listings sometimes live on as active after they have been sold, so it's wise to double-check the status of a home at one of the MLS-linked services.
Trulia is also a portal that pulls data from a number of sources. It's a good place to start researching neighborhoods because it has a set of "heat maps" that let you easily see crime, schools, commute times and amenities such as banks and grocery stores in your target neighborhood. You can even look at the probability of a natural disaster, such as a hurricane or earthquake.
"You can find some of these stats elsewhere, but you can't find it in this format," says Shannon Field, senior director of product management at Trulia.
In addition to searching for homes by ZIP code or city, you can draw a map of the area you'd like to search. Based on your search history, Trulia will send you suggested homes, as well as alerts on new listings or homes you have followed.
Realtor.com is the listing site run by the National Association of Realtors. It's up to date, but it doesn't have as many added features as the other sites. Still, it's a good place to check to see whether a listing is current.
Clarified on 01/07/2014: A previous version of this story failed to include that Redfin provides a complete sales and listing history in some cities.