When you think about it, it's practically impossible to accurately appraise a domain name. It's value really is whatever someone will pay for it. Not even a sale is an accurate appraisal. That said, as far as appraisals go, Estibot is probably one of the most accurate. It uses good data to form a valuation. A combination of metrics weighted against past/current sales. That's about as accurate as it could ever be. I wouldn't waste my money on any of the paid live-person appraisal services, unless you need it for a potential buyer who likes the PDF report you usually get to justify the purchase.