How to add the mashups to Google Maps? With mapplets, mini-applications similar to Google Gadgets. "Mapplets are mini-webpages that are served inside an IFrame within the Google Maps site. You can put anything inside this mini-webpage that you can put into a normal webpage, including HTML, Javascript, and Flash. Google provides a Javascript API that gives the Mapplet access to services such as manipulating the map, fetching remote content, and storing user preferences."
There's already a test page that features some mapplets and they're pretty impressive: real estate search powered by Google Base, distance measurement, movie showtimes, weather, earthquake information and more.
The nice thing is that you can activate more than a mapplet at a time, so you can see the movie showtimes for Shrek 3 in your area and measure the distance between two locations, like in the screenshot below.
Find more about the new "mashup of mashups" from this demo: