Finding comps: 6 sites you won't want to skip (© Comstock /Getty Images)

The Internet holds rich troves of information into which buyers, sellers and gawkers can dig deeply to find real-estate comps.

Sites have different strengths and weaknesses, different tools and unique approaches to listings search. You can test-drive several by doing identical searches on each. Here are a few valuable online tools and hints on how to best use them:

1. Local MLS. The best source for listings and sold information is your local MLS, or multiple listings service, a privately owned database into which sales agents enter property listings and make changes to those listings. Each MLS is owned by a company that collects and publishes market data for a geographic area. Some cities have more than one competing MLS. Some make their listings publicly available online. But it can be difficult to find these and to distinguish them from other sites that use the term MLS for marketing purposes. "This is one of the big conundrums for the Web," says Gahlord Dewald, a technology and marketing consultant whose firm, Thoughtfaucet, helps real-estate agents with social media, search and Web strategy.

  • Try this:  Search for your local MLS using your city name and "multiple listing service" or "real estate information network."
What's your home worth?

2. Realtor.com is operated by Move, on behalf of the National Association of Realtors. (Move is an MSN Real Estate partner.) It offers direct feeds of properties for sale from more than 900 multiple listing services nationwide. "More than half of the 4 million-plus listings on Realtor.com are updated every 15 minutes and the rest are updated once or twice a day," says Julie Reynolds, Realtor.com spokeswoman. Many MLSs also feed sold data to Realtor.com, allowing you to learn of a sale closing within 24 hours, long before the sale is recorded with the county. Counties can take 30 to 45 days to record a sale.

  • Try this: Use Realtor.com's home values tool to click on a city's name and see the average number of days homes were on the market, the average listing price and the average sale price. To find recently sold homes, zoom to the street level and click the "recently sold" tab beneath the map. Learn how many days a property has been listed on the site: Click on a listing, choose "more" in the "Full listing details" box atop the pop-up, then see "listing information," halfway down the page.

Read:  DIY comps: Be your own number cruncher

3. Altos Research offers free information about local market conditions, including average days on market, median price per square foot, number of homes for sale, trends, market conditions and median sale prices. Or subscribe to Altos' weekly "market intelligence" reports ($19/month) for ZIP-code-level data, more statistical and historical analysis and trends in four local price categories. But Scott Sambucci, Altos vice president of sales and analytics, cautions: "We don't intend our product to be used as a sole source of information for somebody to do comps."

  • Try this: Click on the small text link "Free Research" at the bottom of the Altos home page;  enter your city's name in the search bar at the top of the next page (information is not given for all cities). Or, in the "free research" area, click on "map" next to the search box, then click on the red bubbles for detailed charts and data for each location. Send the information to yourself via Facebook, Yahoo!, e-mail, Twitter or instant message.

4. RedFin, a listings site and discount brokerage, retains original listing information after properties sell. Register (free) to see detailed sold listings. RedFin is available in Atlanta, Boston, Chicago, New York, Los Angeles, Orange County, Sacramento, San Diego, San Francisco, Seattle and Washington, D.C. (and soon in Portland).

bingFind more resources for real-estate comps

5. Homes.com shows links to property records for many cities across the country. The site's listing data come from public records, multiple listing services, national franchise agreements and from Homes.com's client brokerages and agents, according to spokeswoman Patty McNease.

  • Try this: Enter your address on this page or click on your state under the heading"Browse National Public Property Records."

6. Zillow.com uses sold information from county records to display recently sold property listings. (On Zillow's home page, click "recently sold" on the lower, far right corner of the big blue search box. Or, on the map view, select "recently sold properties" under "refine search" on the upper, far left; yellow house icons indicate "sold" properties on the map). Data includes date sold, last recorded sale price, property tax information, Zestimate data and photos. The number of days the property has been listed on the site, shown for active listings, is removed when a home is sold, says Zillow spokesperson Jill Simmons. Zillow gets listing information from brokerages and MLS companies.

  • Try this:  See David Gibbons, Zillow's director of community relations, show how to find comps using Zillow in this YouTube video. Subscribe to e-mail alerts of recent sales as they happen in your neighborhood: Enter your home's address into Zillow's search bar and, from the property's page, click "get e-mail alerts" on the far left.

Keep in mind: Many (not all) online listing sites make their money serving real-estate agents. In addition to displaying property listings, they sell sales leads to local agents. This means that they may want to steer you to agents who have paid for promotion on the site. Think of it as part of the deal: Those agents are paying for your free access to MLS listings. If you're looking for an agent with expertise in your neighborhood, this is a good thing. If you want to search independently, you'll learn to work around these features.

Hint: If you want to keep your e-mail address private, establish a separate e-mail account with a free service such as Hotmail (owned by Microsoft, parent of MSN) to use in your comps search.