Zestimate from Zillow and its sister site Trulia is the most popular online home value estimator. Zillow's home value estimates are often considered to be the most accurate; however, this depends on location and available data. Known as “Zestimates,” they have a national median error rate of 7.5 percent for out-of-market homes and 1.9 percent for listed homes. This means that Z estimates are within a range of two percent of the final sale price of half of all properties on the market.
Zillow is the number one website where many home buyers and sellers alike turn to. It's one of the biggest and most popular sites out there and it's very easy to use. Zillow never claims to be 100% accurate. If all homes within a six-block radius are very similar to each other, a Zillow estimate will be much more accurate, perhaps within 10%, because there aren't enough specific variances to derail it.
In other cases, such as older neighborhoods with many homes that have been improved in different ways, you won't be that close.