Google Maps adds a new dimension to buildings in the street maps of more than 30 important cities from the US (like New York or Boston) and Japan. Frank Taylor from the unofficial Google Earth Blog argues that they're not yet 3D. "You can't rotate or tilt your view, so it's 2D. But, the buildings have shaded 3D-like projections from an angle so you get an idea of height and shape of the buildings."
This is, in fact, Google Earth's building layer. To see it, go to the layers sidebar and enable "3D buildings".
Google Earth's layers contain a lot of useful information that enhance the satellite imagery: road names, airports, parks, populated places, pictures and more. The cool thing is that they reside on Google's servers, so you don't need to update Google Earth to see new information. Google Maps could become much more useful if it included these overlays.