Seattle Needs a GarbageScout
There are a ton of great culture mashups riding on top of the Google Maps API. Lifehacker recently featured GarbageScout, a site that ties interesting junk to the location it was found in New York. While New York certainly has an advantage in population density, this could be a cool way to map virtually any city. Connect it to Craigslist and there's a new economy in trading old junk (not that eBay isn't already doing quite well).