There are quite a few, like http://www.strategicfirst.com/ , but they don't really show anything meaningful and assign seemingly random values to statistics such as the PageRank of your site. Only use these for fun, not for business.
To get a real idea on what your website is worth you need your Pagerank and income per month, along with uniques page views etc. Depending on the type of niche most people will usually pay 6-12 months total revenue for your website. So say your making $30 per month 6*30 = $180 at the minimum.