NASA has an Open Source project called World Wind and allows you to zoom in to high resolution details of cities and landmarks around the world. It's free but due to growing popularity, getting access to the server is a hit or miss.
The alternative to this is Keyhole and when Google purchased the company, the first thing they did was lower the subscription fees to $30 USD. A much better price.
Keyhole seems to be much faster, no server timeouts and the detail is much crisp. I guess you get what you pay for. Not every city is covered but many of them have amazing image resolutions.
