My favorite mashups were www.lastingnews.com/maps/cia_prisons.html, www.1001seafoods.com/fishing, and www.beermapping.com.
All of these sites combine google maps with things that I find interesting. I know that I will use 1001 seafoods to plan my next fishing trip. I am always looking for the best spots to catch a lot of fish. I like the cia prisons site because it is far out and probably not based on great evidence. I also like the bar mapping site because I like to drink beer. I found some of the other mashups rather boring. I have no desire to buy a house and I never use ebay or amazon.