I won't tell you which one (and it's not in my profile), but it's the closest you can get to an accurate value.
Should you arrive at my valuation site, do a valuation, read what it says and throw that figure away!
No valuation site is going to tell you anything useful about your site's value. In fact, there isn't one value for your site - every potential buyer out there likely has a different figure in his mind as the maximum price he'll pay for your site.
If you're looking to sell your site find out the best way to get that buyer who has the highest valuation and the best way to get him to bid to his limit.
oddsod nailed it. your site is worth whatever someone else is willing to pay.
one rule of thumb I've heard is: take the site's annual profit, and multiply by three.
that's profit, not revenue. This takes into account the technology/marketing costs, and any staff or expenses necessary to maintain whatever it is that the site does.
When you're setting an asking price, start with a number proportional to what the site is currently earning, and maybe bump it up if it's sitting on a killer domain, or if it comes with a massive mailing list, piles of .gov or .edu backlinks, solid PR, other assets, etc.
Online businesses are valued like many other businesses -- based on a multiple of key metrics.
The major online metrics are profit, revenue and audience. A lot of businesses are valued on 1-5 times revenue. A typical value based on profit is 15-25X. Growth rate is a big factor on the final multiple.
Go to Yahoo Finance, enter GOOG for the ticker symbol and then click on Key Statistics in the lower left. Google is currently valued at 5.5 X revenue and 21 X recent earnings (both high because of growth rate).