There are lots of tools, but I have found http://www.dnscoop.com to be the best so far, as it uses various factors to determine an approximate value.
A funny story about it is that I discovered it, and talked about it on some forum, without posting it on Digg. Wouldn't you know my friend CodedChaos posted it and got all the digg frontpage glory for it - that really set the site off, and made it what it is. It went from pr0 to whatever it is now (5 or 6) in like a month. Too bad I don't own it!
I stumbled upon that site when I was searching for domains. Very nice, but the text link ads section gives a misleading estimate I think. It gives Page rank 0 with no traffic sites a value of $5/link while other sites have values of less than $5 when their traffic/visitor count is higher. I wish it were to include a domain name potential value estimate in there too