Zillow's chief financial officer, Spencer Rascoff, says that if he were getting ready to sell his home in Seattle, he would refine the estimate by including a new garage roof and making sure that the comparables are, in fact, comparable. "Then I might well print my Zestimate out and distribute it to prospective home buyers."

The site is supported by advertising and doesn't intend to compete against real estate agents, Rascoff says: "What we're doing is trying to put tools and information into the hands of the consumer. We expect most people to seek out the expertise and advice of a professional real estate agent."

Zillow was preceded by two Web sites that do compete against real estate agents. Homekeys and Redfin provide estimates of home values and real estate brokerage services, too. Homekeys represents buyers and sellers and has been online since March 2005. Redfin has been posting home price information for a couple of years, and early this year started representing buyers in real-estate transactions.

Whereas Zillow.com is national, Homekeys and Redfin are local. Homekeys' scope is limited to three South Florida counties -- Miami-Dade, Broward and Palm Beach. Redfin so far is limited to King County, Wash., where Seattle is.

When Zillow went online in early February, bloggers and reporters criticized it for some inaccuracies and gaps in data. Rascoff acknowledges the problem and says, "We'll look for ways for individual homeowners to correct that information for us."

Track asking prices and amenities at Realtor.com
Realtor.com, a Web site run by the National Association of Realtors, doesn't estimate home values, but it lets you know how much your home-selling neighbors are asking, and it gives details about the homes for sale. A Bankrate reader in Gainesville, Fla., says he checks the Realtor.com site when homes in his neighborhood are listed on the Multiple Listing Service, or MLS. "Then I look in the public records a couple of months later to see what it really sold for," he says.

He frequently updates a spreadsheet with details about houses in his neighborhood -- asking price, sale price, square footage, price per square foot, and even distance from a major road. "There's no effect on price!" he says in an e-mail. A number-cruncher by profession, he is comfortable using a spreadsheet.

Another Bankrate reader, Troy Kleve, peruses MLS data on the Web site of his local newspaper, The Sacramento Bee. "If you check the site frequently, you can stay on top of any price reductions and see how fast homes are selling after they hit the market," he says. Visitors to Realtor.com can do the same.

There is a drawback to Realtor.com: You can't search by address. When you spot a neighbor's new "for sale" sign, you can't just run to your computer and find the listing immediately on Realtor.com. Instead, you have to search by city or ZIP code and then click on page after page of listings until you find your quarry. Once you do, just write down the MLS number and use that as a shortcut whenever you want to check up on the listing.

Step away from the keyboard and visit comparable homes for sale
It's not enough to let your fingers dance across a keyboard and mouse. You've got to expend shoe leather and tire rubber by visiting comparable houses that already are on the market. Someday those houses might be your competition.

Linda Lenox says she does a number of things to monitor the climate and temperature of her Bay Area neighborhood. She peruses the classified ads in the newspaper, gathers fliers from "for sale" lawn signs, attends open houses to chat with real estate agents, visits new housing developments, keeps real estate advertisements that arrive in her mailbox, and looks at real estate Web sites.

"I then look through these fliers and papers once a month or so to get the feel of prices and how long the houses are on the market," she says. "I don't tabulate anything, but I do get a feel of what is happening."

That's fine. Keeping track of your neighborhood's real estate is an art, not a science.