Available as a developer tool since 2008, Google has now officially opened up Earth View in Google Maps to everyone. You need to install the Google Earth plugin for your browser but once it’s up and running, you’ll have access to three dimensional views of certain places of interest (search for Stonehenge and zoom in, for example).
You won’t get 3D views of everything (my building isn’t in 3D but plenty of the larger buildings around me are), and a bunch of important landmarks are available. Everything’s pretty quick and responsive, too, assuming you’ve got a decently powerful computer.