The estimates you'll find on various websites, are just that, estimates. While they may be worthwhile as a general "guide", I have seen them off by tens, sometimes hundreds, of thousands of dollars either way....in terms of what they would realistically sell for. There are lots of factors they consider which have no bearing on your property's value, and conversely, don't factor in a lot of variables that do.
Case in point - let's say one of the popular websites estimates your home at 200k. How would that website know if you A. went through the house with a sledgehammer and destroyed the interior last night, or B. finished the basement, put an addition on, or completely remodeled the kitchen and all of the baths?
I'm sure if you invest 20-30k into remodeling your home, you'd want that investment factored into the selling price of your home. These estimates have no way to ascertain the condition, upgrades, or defects that the property may have, and should play a very small, if any, role in determining true market value. Using comparative analysis of similar properties sold in the last 6-12 months is a much more accurate and reliable way to determine value.