Satellite, aerial photos and map sites
 

Google Maps
Provides maps, satellite images, directions, landmarks, businesses and, uniquely, "Live View" shots from ground level across the US. See also Bing & Yahoo
Mapping Social Media
Geofeedia & Echosec plot Tweets, Instagram pics etc. Onto a map. Geosocialfootprint and Tweetpaths display the locations someone has Tweeted from.
 
Open Street Map
As far as road maps go, this has got to be among the most complete. If Google & Bing don't list a road, look here.

Geohack sources
Nice list of satellite & map sources around the world

Historical UK Aerial Photos
See what your house looked like in the 1940's. See also Britain from above.
MyGeoposition
Find geo-coordinates, copy them and use with Twitter's geocode: search term to find Tweets from that location.
Track Aircraft
FlightAware shows the progress & routes of aircraft, plotted onto maps.
Track Shipping
MarineTraffic shows the progress & routes of ships, plotted onto maps.
  Others