Look Local by Idelix
Someone forwarded me a great site comparing Virtual Earth, Google Maps and Yahoo Maps. You can see it here: http://looklocal.idelix.com. This is a fantastic site! I love the mini-window which provides contrast between your focus area and the rest of the world around it. The zoom factor is sweet allowing you to zoom up to 4x within the mini-window. I also like how you can grab and move it OR lock it and have it maintain it's position on the map. Not only can you compare maps, but you can compare local search results from the different companies. Nice job putting this together Idelix!
Now, what Idelix doesn't do is show off capabilities that are available in certain platforms that aren't available in others. For example, there's no Bird's Eye imagery or 3D integration which Virtual Earth provides. Also, they don't take advantage of the unique pushpin capabilities available in Google Maps. Still, a great site to reference.