I just discovered that Google Maps now covers the United Kingdom, and practically wet my pants. Their map of London is very, very, very nice. Note the prominence given to Tube stations.
sfllaw and I decided that it would be cooler still if, in addition to map and satellite, they added some sort of transit view for major urban centres, which would show subway and bus routes. (Bonus points if they can do transit directions showing transfer points and whatnot!) We've submitted a feature request.