http://earth.google.com/ I've been using this for a while and thought will share the same here. - I downloaded when a friend recommended it and then spent the rest of that week playing around with it. I went on a trip to Malaysia some time back, and took a bunch of photos up. Not only did I find the exact mountain on google earth, but I was able to position the google earth 'camera' so that it almost exactly matched my own photographs of the place. Totally amazing that they have the whole globe to such detail.
Places like say New York and other big US cities, it is so Hi-res you can read the road markings and see individual people (well, the tops of their heads).