The Street View feature from Google Maps is a fantastic tool which I use many times a week. It lets you explore places you haven’t been before, scout out locations before heading out to visit them (ie, exact location of a restaurant) and it lets you see changes that have been made to an area.
Since launching in May 2007 in select cities, Google has expanded their pictures to most of the US, most of Europe, an other countries around the world.
Here you can see that most of the US, Mexico and Canada have Street View images available, in blue. The blue circles are user submitted pictures in areas not yet covered by the cameras.