DoubleClick Ad Planner by Google
http://www.google.com/adplanner/

Refine your online advertising with DoubleClick Ad Planner, a free media planning tool that can help you identify websites and target customers likely to visit with these features: a) Define audiences by demographics and interests; b) Search for websites relevant to your target audience; and c) Access unique users, page views, and other data for millions of websites from over 40 countries. Easily build media plans for yourself or your clients. Create lists of websites where you’d like to advertise. Generate aggregated website statistics for your media plan. This has been added to the tools section of Research Resources Subject Tracer™ Information Blog. This will be added to Entrepreneurial Resources Subject Tracer™. This will be added to eCommerce Resources Subject Tracer™.