Just a few years ago, appraising your own property was as simple as checking out how much the house down the street sold for and adding a few thousand dollars for good measure. Not anymore.

With homes in many markets lingering unsold for months, recent sales of neighboring properties are hardly the accurate indicator they once were. And in areas where the majority of recent sales are foreclosures snatched at attractive discounts, potential buyers often get the wrong idea of just how low of a low-ball offer they can convince an owner to accept.

To get a better handle on their local markets, more homeowners and potential buyers are turning to real estate web sites for their appraisals. These sites prompt users for a residential address and then spit out a price or range of prices indicating how much that home is worth. Visits to Zillow.com came in at 4.1 million last month, up 55% over the same time last year, according to Nielsen. Trulia, which offers listing information, saw a 37% increase over the same period, and Yahoo Real Estate, which offers home value estimates from Zillow.com and Eppraisal.com, saw its traffic rise 20%.

Of course, popularity has never been a good measure of accuracy on the web, and the traffic numbers for these sites do not tell the whole story. Zillow.com says that its estimates can be off by as much as 34% in Dallas and nearly 20% in Detroit. (The site's median margin of error for all 50 states is 11.8%.) HomeGain.com, which started offering a so-called instant valuation tool in 1999, took it down less than two years later because of complaints about the tool's inaccuracy. The site brought it back in 2006 and now offers a range of values, as well as referrals to real estate agents who can provide more accurate prices.

"It's almost like alchemy, trying to get there," says Louis Cammarosano, the general manager of HomeGain.com, of finding the formula that predicts property values accurately. "There are too many things you just can't know and the only way to know them is talk to a real estate agent."

So how do these sites determine property values? They start by collecting as much property information as possible from public records and Multiple Listing Services, including a home's age and size, the number of bedrooms and bathrooms, and the sales history of comparable homes nearby. Then, they run all that data through complex algorithms, known as automated valuation models, or AVMs, to calculate how much the property is worth.

Lenders use AVMs to determine the risk of default across their loan portfolios -- a strategy known as risk management. And appraisers use AVMs when they have to value large numbers of properties for tax purposes, says Leslie Sellers, a Knoxville, Tenn.-based appraiser and 2009 president-elect of the Appraisal Institute, an industry association.

But AVMs are rarely the only information used in individual home sales, which require a much more in-depth look at a property. For example, an AVM may take into account the number of bathrooms in a home, but it would ignore the quality and size of a bathroom, as well as its fixtures, Sellers says. So even if a house identical to yours down the street may have sold for $200,000 two months ago, your home may be worth more after a five-figure investment in a kitchen renovation.

In today's market, appraisers are also finding more need to make adjustments for so-called "market conditions." If the seller of a nearby property was highly motivated say, they were behind on their payments and accepted the first offer that came their way an appraiser would adjust the value of your home higher than otherwise calculated by a less-sophisticated AVM. On the other hand, if the seller made concessions they paid part of the buyer's closing costs, for example the value of your home would be adjusted down.

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A problem less easily corrected is that the data used by AVMs and home valuation sites are often wrong. "They're really at the mercy of public record, and public record is flawed," says Jonathan Miller, the president and CEO of New York-based Miller Samuel Real Estate Appraisers. For example, extensions added to houses are often left off the public record so the square footage and number of bedrooms or bathrooms can be wrong. (To address that problem, Zillow allows homeowners to "claim a home" and add in such details. An owner's estimate, is then calculated and published alongside the property's Zestimate. As of July, 11.8 million homes out of the 70 million listed on Zillow have had their facts updated by the home's owner or listing agent, spokeswoman Amy Bohutinsky says.)

Another caveat is that home sales data are not part of the public record in 13 states, increasing the algorithms' room for error. (Those states are Alaska, Idaho, Indiana, Kansas, Louisiana, Mississippi, Missouri, Montana, New Mexico, North Dakota, Texas, Utah and Wyoming.) To collect data for these states, the home valuation sites can partner with data aggregators or multiple listing services. For example, CyberHomes.com receives title insurance data from Fidelity National Financial (FNF), of which it is a former subsidiary.

Of course, none of these problems may matter in the end. Knowing the exact value of a home may not be the key to being a successful buyer or seller in today's market. On the other hand, arming yourself with as much information as you can and surrounding yourself with knowledgeable professionals is critical, and that is where these sites can help.