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Without knowing exactly what type of free advertising you have in mind it's a little difficult to answer specifically. However you might want to take a look at the Tested and Proven site. They regularly post results on a group of higher performing free ad sources. The site cost nothing to join and there are no upgrades for it... http://testedandproven.biz/?rid=310
Check out Steven Ackerman Advertising. I have an extensive choice of different kinds of advertising, many of which I have written reviews for.