Auto appraisal sites are all a joke. You can always list your domain for appraisal on sites like DP, Namepros, Domainstate & other domain forums
Yes estibot is one, although I highly doubt the accuracy of it. Better to ask a professional on a forum than rely on something like estibot.
I wouldnt bother with any appraisal sites! just ask a few people you know, or post on the forums. Although dont always rely on what people say, so try and stick to your thoughts.
The website in my signature has an application that provides an potential monthly revenue estimate. Assuming you developed a site, you could then extrapolate a multiple based on the revenue. Hope this helps.