In competition with Google, Microsoft have released their own mapping system. Live Mapping with a recent beta of Virtual Earth 3D. Virtual Earth 3D ...  potentially for me gives it a little bit of an edge over Google Earth. However, and a BIG however ! You cannot do reverse Geo lookup on an address ! You can perform "searchs" that will return a result set, but you can't directly translate a location string into a set of long/lat coords, which is very annoying. Not to mention ...

The JavaScript implementation of the API doesn't work with Opera, part's of the API don't work with FireFox AND the map controls do funny things when you attempt to float the DIV that contains the map.

It's extremely buggy at the minute. I must admit I really love the 3D feature but I'm sure Google will be following suit shortly.

Further Reading
Microsoft "Virtual Earth" API
Google Maps API