Domain names are very illiquid and there are lots of different ways to define worth. On the low end, there is this price: how much could I sell it for tomorrow in a no-reserve auction? On the high end, there is this price: how much could I sell it for if I spend a year cold calling and negotiating with potential end users?

Almost any domainer will tell you to completely ignore automated appraisal services like Estibot. Most will also tell you to ignore professional appraisals; they are better at estimating auctions prices than anything else.

Look for similar names on AfterNic, GreatDomains, Pool, or other domain reseller sites. But you will not really know until you try to sell it - just set a high reserve to make sure it does not go for a song. Note: This assumes a legitimate domain - if you have a trademark - you will lose it and it will cost you a LOT more in legal fees.

Try using Sedo's (sedo.com) appraisal service. They're fairly in-expensive and they're also reasonably accurate.