Companies that sell banner advertising opportunities cross multiple websites and cross channels are considered “ad networks.”  For instance, you can impressions at a CPM rate, but appear on several websites in their network instead of just one.  This form of media buying can be very cost effective if you know the traffic potential of the sites in the network (top tier sites).  Because the network buys these ad impressions in bulk quantities, they can pass the savings down to the advertiser.  Be sure to ask when you buy this type of media what is the traffic ranking (Nielsen Net Ranking, ComScore or Alexa) of the sites and sample listing of the sites in the “channel” you’re considering.  You want to make sure you are appearing on top tier sites, not micro sites – it’s quality over quantity. You can also target your ads to run on different verticals, or channels, based on your offer.  Popular channels include finance/investing, news, sports, and politics.  Lists of sample ad networks can be located at http://www.imediaconnection.com/content/7911.asp and http://www.clickz.com/showPage.html?page=resources/adres/ad_networks.