According to a research done by Nielsen//Net Ratings in July of this year. 54 percent of online ads were placed into e-mail programs. (This is the little banner ads that appear in your e-mail or tagline ads at the bottom of messages in your e-mail).

After e-mail, the next-largest categories for ad placement were General Community (12%), Portals and Search Engines (9%) and other (9%). 
However this does not correlate to revenue generated. Email ads tend to generate less income because often they not targeted ads. However if you have a focus audience, this could be leverage by being specific in the ad placement you send out to your subscriber list. This can significantly increase the return in your web business.

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