Following are my favorite Google Maps Mashups:
2. Twitter Spy - See where people are microblogging. It should have geographic boundaries, I could do it myself, but I would have to study Twitter's API.
3. Street View Immersive Videos - Like Google Maps Street View but now as a Video, use the mouse to rotate the view.
4. Free real state mashup - find houses and apartments for sale in Brazil, search by size, location or price, for free.
5. Geolocated voice messages - I think that is a great Idea, but it has to be specific to achieve greater success.
6. Paris user contributed map - better than Wikimapia because markers are simpler than polygons, categorization is good for searching and the side bar is easier to locate.
7. London Profiler - Nice looking data viewing!
8. WikiCrime - A wiki for crime reporting and locating.