http://www.google.com/analytics
http://www.statcounter.com
Edit: Sorry, thought this was for a site you personally administer. Closest I can find is the Alexa Sparky Firefox add-on:
https://addons.mozilla.org/en-US/firefox/addon/5362
Install it then use the "Traffic details" option. Unfortunately there's no real numbers available - just web traffic percentages.
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