You can use Project Wonderful to know exactly how much traffic a site gets. I will explain how immediately after this:
I’ve just read a post on my old blogcatalog buddy’s blog, about how Entrecard is making good money with Project Wonderful ad spots.
Just to recap his article:

That is why.

Project Wonderful’s says in its intro that they are all about fairness and transparency.
Indeed! That same transparency is what I use to know what traffic a site that has a Project Wonderful ad spot gets.
Go to the site you are interested to spy on. If you look below the Project Wonderful ad spot, it says “Your ad here, right now: $0.00“. This is a link, and if you click through you will get to a purchase page with a monthly graph showing the site’s Unique Visitors and Page Views. It shows the stats for the page the as is place on, but basically it would reflect the sites overall traffic, because webmasters would put it on the most visited page, to get the best prices.
So now you know. Use it wisely.
- Alex
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Hey Alex!
I think you are doing a good thing with publishing this article. You had already made a reference on the spying technique in the past but it is good to see it over here
I guess project wonderful, apart from being cheap for the advertisers also is good if you are not yet up for the hussle of finding advertisers, …. but true, it is not the most profitable way to use the sidebar space…
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It is tempting because of its simplicity. I know…
But many pros say that it is relatively easy to find direct advertisers. Just a little bit down the road for us, after hitting a little more daily traffic.
….. you hit the nail on the spot…. “hitting a little more daily traffic” LOL but meanwhile… I´ll just go for the simplicity, working on too much other stuff right now
take care
ps …. again, I am so sorry about losing the comment, I know you had to go through some trouble there
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Is there anyway to tell how much traffic a site gets without project wonderful?
See if the site has a SiteTracker.com button anywhere.
This service too enables to view the site’s traffic stats, uinless specifically chosen not to by the owner.