Terry Farnsworth, Broker, Crystal Lake, IL Molly,
Please do not trust an online estimate as a definitive valuation - regardless of the website. Review enough of these generic valuations, and you'll find that they can vary by as much as 40-50k, and sometimes more. Additionally, these websites have no idea of the condition of the property. Whether the walls of the property are mold-ridden and there are holes in the roof, or if it's been completely rehabbed with top of the line materials - these estimate don't change or account for it in terms of value.
Would you blindly trust one of these sites when pricing your home, or putting in an offer on one - knowing that there is a chance you are under or overvaluing the property by even 10K? I hope not.
I've found that people tend to "trust" these valuations when it's in their favor to do so (i.e. they want to sell and the estimates tell them it's worth more than the professionals do), but discount them when it's not (property is estimated below what they want to list it for).
Please contact a few real estate agents in your area, if you want an accurate valuation of your property's value based on recent comparable sales.