oseta

When Google is not able to serve relevant AdSense ads on a web page, the system will display Public Service Ads instead of the regular ads to promote organizations like Red Cross, Direct Relief, etc.

Unlike the regular AdSense ads, web publishers do not get paid for clicks made on Public Service Ads and even Google doesn’t make any profit by running such ads.
In their own words – "PSAs are non-profit organization ads that are served to pages when targeted ads are unavailable, or when Google is unable to gather content from the page."
Wikipedia too defines a public service announcement as "a non-commercial advertisement broadcast for the public good."
Google launched their own 2008 U.S. Election site in August that, like a portal, links to information about US elections available on other Google Sites like YouTube, Knol, Google Maps, etc.
Now if you happen to spot an AdSense PSA on some site (like here), chances are that it will be about Google’s own Elections website as in the screenshot above.
This is surprising because this "ad", though a PSA, doesn’t fit in Google’s own definition of PSA (non-profit organization ads) nor that of Wikipedia (non-commercial advertisement).
Here’s what ABC News has to say about Google’s elections page – "the new applications are part of their effort to enhance the search giant’s involvement in U.S. politics and, ultimately, to become as central to the electoral process as Google and YouTube have been to media usage."
All the unfilled AdSense inventory is helping Google promote their elections page but am not sure if these ads classify under PSAs or "free advertising" because Google does have "commercial" interest here.