The current state-of-the art in Online Maps has to be Google’s excellent online map service:
Google Online Maps Simply type in your desired destination and a very detailed street map will appear for your use.
Yahoo Online Maps is currently out in beta and offers another excellent choice for those that need detailed street address information.
Mapquest offers the de-facto standard online map for driving directions and trip planning.
Anywho also has a online map resource available. Less detailed and helpful than the others, but still useful.
Satellite and Arial Maps can be found over at Google too. Get a bird’s eye view of just about any place in the U.S.!