The Google Adsense program is a great way to earn some extra dollars for your website.
Sometimes for various reasons the normal pay-per-click, PPC ads are not available from the server and what will be displayed is a Public Service ad (PSA).
Consider replacing these ads with your own advertisements. Sure you can get other feeds to substitute the Adsense feed, but in the long run you may not be maximizing the website for the best sales tool available to your business.
By crafting your own advertising you can actually be building up a number of advertisements for future PPC AdWords campaigns to run for your company. This way you can get an actual feel of how the ad will look on similar sites.
Some business folks may have an aversion to displaying advertisements on their web site to begin with. The argument is that if someone has come to your B2B or B2C site why in the world would you want to send them away. While this argument certainly has validity from a quick look, another argument could be made. With competition blocking filtering and displaying ads that are related but not directly in competition with your business you are actually providing a service for your Clients and Customers and who are much more likely to return in the future. Additionally, if your product or service is strong then you will get the cream of the crop staying with you.
Many times the normal PPC advertising feed will not display the standard advertisements because there simply is not enough content in the pages for the algorithm to figure what is the best ads fit. Other times, there might not be enough advertisers that are bidding on the keywords in the page. But no matter what the reason take the time to generate some advertisements for your own business. It will pay sales dividends in the end.