In the beginning was Google Maps. They had a nice interface to street maps. Then they added satellite aerial photography to the maps. Then they released the programming interface to the world. Then things got neat.
Many people have created cool toys using this maps interface. For example, Gmap Transparancies puts a street map on top of a portion of the aerial. BeenMapped lets anyone point out cool spots to everyone else (like a Stealth Bomber sitting at Edwards Air Force Base). There is also a pedometer and a weather map and a map of WiFi hotspots.
Slashdot just pointed out a project called Greasemap. This uses the power of Greasemonkey, which is an extension to Mozilla Firefox (the coolest web browser in town). Greasemap reads the webpage you're surfing, and if it finds an address, it fetches the Google Map of that location.
For example, when I view this blog entry, I see a map to my old house, which is 352 Henderson Dr., San Jose, CA 95123.