[UPDATE 5-September-2007: Check out Google Earth 4.2's new built-in flight simulator]
Imagine running Microsoft Flight Simulator and Google Earth together. Imagine seeing your simulated plane, and other auto-generated planes, appear within the Google Earth. Turn on the roads layer, the airport layer, etc. and you can see all kinds of data associated with where you are flying that the Flight Simulator does not.
These are just the simple by-products of a free application announced in July by Juergen Haible (http://www.elbiah.de/). His application is called MyFSGoogleEarth and you can find it, along with instructions for installing it here. This is a program for the more serious Flight Sim user and requires at least the free version of FSUIPC.
As Google Earth becomes even more useful (in terms of data and interface), I expect there will be even better interfaces between GE and MS Flight Simulator in the future (despite the rivalry between Google and Microsoft). Great job Juergen!