There are quite a few that I have seen mentioned here:
redfin.com cyberhomes, zillow (although I have seen zillow be way too high and /or not use reliable comps. I also found a new one recently eppraisal. These might give you a rough idea what houses are going for in a specific area. I am sure the realtors and other subject matter experts here like TG have even more recommendations for you. Happy hunting.
