Google Earth's 3D view, of course, but Nokia just might have a leg up in some respects. Unfortunately, it's still staying mum on exactly how it all works, but you can dive right in and start exploring for yourself at the link below.
[Thanks to everyone who sent this in]
Update: Nokia's now finally gotten official with this on its blog, and revealed that C3 is responsible for the impressive 3D mapping technology. Hit up the links below for some additional details, and head on past the break for a quick video demo if you need a bit of incentive to install the necessary plug-in.