All offer some form of automated "free appraisal" that isn't worth what you paid for it. Compare the results between the various "appraisals" and you'll see wide discrepancies.
Of these, dnscoop may be the best of the worst (as they at least explain what the basis of these numbers might be) and leapfish.com is the worst of the lot IMHO. None are reliable.
There does seem to be a bias in the results toward overestimating values of names, based largely on keywords and traffic/pagerank stats. Perhaps this is because these "free" sites make most of their money by advertising companies wanting to push text links onto pages. "Here, deface your site with this, you'll make money fast overnight..."
They're great for comic relief, but all have limitations.
[This post has been edited by c_a_b_kingston_ca on Jul 13, 2007 12:26pm.]