Google added a new view to Google Maps, basically turning the Earth into Hyrule (or Alefgard, since it looks just like Dragon Warrior). Everything is tiled and pixelated, with little castles replacing the cities.
First watch this promo video:
Then go to http://maps.google.com and click on the Quest button in the top right corner. Try street view too.
Also, during your quest look out for hidden items! Things like Area 51, Google's offices, Churchill Downs, Nessie, random monster encounters, and much more.
Of course there are already websites listing the hidden items. Ben Garvey has an ongoing list at his site, and there is also Google Maps 8 Bit Easter Eggs on Tumblr. Let us know in the comments if you find anything really cool.