This application allows ArcGIS to open a Google Earth window of the same coverage and keep it synchronized. It's free and apparently benefits from the ads that appear at the top of the map.
The Google Earth window is updated as you move or zoom in on ArcGIS ... and it also works in reverse, navigating in the Google Earth window.
How to download it
To access, you need ArcGIS 9x, and download it from this site.
How to install it
Once downloaded, unzip the .zip file and run the Run _install.bat file
How to activate it
When running ArcMap, in the top menu, select "window / google"
This video shows how it works, and in this post of the Gabriel Ortiz forum show you how to break the coconut if it gives installation problems.