PSA's (aka Public Service Announcements) still exist, but are probably not as noticable to most in our media saturated environment. As you might expect, cartoonists and cartoon characters are often a part of them. Of course, they also serve as an advertisement for the cartoon itself. Here's a current one that I walked past for a couple of months (excuse the cell phone quality):
This Blondie panel from 1970 was in a newspaper in the National Museum of Health and Medicine:
These comics of the Pink Panther, Blondie, Beetle Bailey, and Mutt & Jeff all come from Navy medical newspapers or newsletters.
Speaking of Mutt and Jeff, cartoonist Al Smith drew it for about 50 years. Here he is entertaining patients at a 1971 visit to Bethesda's National Naval Medical Center.
Caricaturist Jack Rosen visited Naval Hospital Orlando in 1979.
Of course, sometimes an ad is just an ad. This US Postal Service Mover's Guide Official Change of Address Kit, January 2013, has a Disney advertisement, and is available right now from your local post office.
These are minor footnotes in a larger history of comics, but hopefully enterained you briefly.