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Your best bet is to use Alexa and Google PR to guessimate. Or if you are really sneaky have them place an image on their site that loads off your site.
However, I've seen some figures and I'm not too sure how accurate they are. Also, the system that I saw monitored a fairly skewed demographic, so the results were not entirely representative of overall traffic.
You usually won't luck out if you're trying to figure out what a specific site's traffic is, but if you dig, you can usually find stats on various sites in your topic area.