Google has announced AdSense support for Google Maps hosted on your site. In other words, if you host Google Maps on your site you can get "contextual" ads in them, related to the location being featured. Any user of your site clicks on the given ad in the map, you make some money. Setting this up is relatively simple, since all you have to do is include a few extra lines of javascript in your script controlling the map.
To see the result for yourself, go to my map at http://ajax.marcocantu.com/maps, featuring a map of a classic Delphi sample database table, and have a look. Notice that since the map is quite a large view (the entire US) the ads are quite generic. But if you zoom into any area, they start getting very specific. For example, by purely zooming I got near St Louis:
Moving over an "interesting road" in San Francisco:
You still get ads related to the hosting site (including Delphi components for mine), very generics ones (I kept getting a trip from Rome to Bali while browsing the US map from an IP address in Italy, but most are specific to what you see... or getting close).
Sounds extremely interesting to anyone already hosting maps or planning to do so. Try for yourself with my map following the link above or any other ad-powered map.