The only site I know of that does estimate for property value is Zillow.com. Every time I get a question like this, I go to Zillow to get an estimate on my own home, and it is still not accurate as of today.
I think the reason why is because Zillow does use the most recent sales around the neighborhood for a home and try to estimate the value of the house. The only problem is that it takes the hard facts (# BD/BA, SF, acreage, etc) but does not account for upgrades, views, location (open space, hillside, etc), unique characteristics (horse property, cul de sac, office/loft vs. bedrooms, etc); all these can make a big difference on the price of the home.
So, zillow might work for cookie cutter homes in a cookie cutter development, but will have a difficult time coming up with values for homes that can be quite different from each other within a neighborhood. Those will need human interpretation for a more accurate value.
If you notice next to the Zillow logo is the term "Beta". Beta means a first software release and is usually distributed to users as a way to learn the bugs of the program, get new ideas and improve its functionality.
Zillow is aware of its inaccuracy and is currently taking steps to improve.
As a realtor, Zillow is a problem because buyers/sellers see it as golden and are not aware of its inaccuracy.