When AdSense is absent without permission
No matter how hard you work to optimize your page, there will be times when Google just can't figure out which AdSense ad to deliver, so by default, it will deliver a PSA (Public Service Announcement).
Now I have no problem with charities, but I give to the ones I decide to donate to. Since I don't have a non-profit license of my own, the goal of my website is to make money and I rely on Google AdSense revenue to help pay my bills. Someday I want to fund my retirement too, so I can't afford to have no income public service announcements appear on my site.
The good news is that Google understands the basic greedy nature of the human being, which is why it provides us with an alternative to donating our valuable web space to charities. That alternative is known as AdSense alternative ads.
Oddly enough, this feature allows you to allow Google's competitors to enter your site. Don't worry, Google allows it with your eyes wide open. They even tell you how to set up the alternate ad code to work on your site and let you do it right in your AdSense dashboard.
Once you add the code to your site, Google will pull ads from whatever services you have defined instead of offering a PSA. Google will do it even if those ads come from Yahoo, Overture, or your grandmother's attic.
This goes a long way in ensuring that you never miss an opportunity to monetize the time a visitor spends on your site. How nice it is for Google to give us that opportunity.
Who are you going to choose?
Ah, that's the big question. Most people head straight to Overture or Yahoo, but there are other fish in the sea worth considering. In fact, some of these fish make their living almost exclusively by running replacement announcements for public service announcements. Run this search (http://www.google.com/search?client=firefox-a&rls=org.mozilla:en-US:official_s&hl=en&q=google+psa+alternatives&btnG=Google+Search) on Google and you have many options to choose.
Why bother?
Sometimes Google doesn't have any ads in their inventory that match your site's keywords. Sometimes Google gets confused and can't figure out which ads to run, so it takes a PSA ad.
Google also has an unavailable list of what it calls "stop words." When the Google AdSense spiders detect these words on your page, they automatically trigger public service announcements. Some of the more commonly known words include severe profanity (think 7 Words You Can't Say on George Carlin TV), as well as other words that may be quite . At least we BELIEVE it goes on and on, but no one knows for sure outside of a few trusted Google employees.
But no matter what the reason is, you don't want non-revenue-generating ads to run on your site. There is no excuse when Google makes it so easy to keep your income flowing.