Ning user Jon Aquino has created a mashup of Google Maps and Yahoo! Maps (both AJAX and Flash versions). Drag the map around in any of the 3 panes to get a comparison of what the mapping data is like for that particular point. Zooming works in aggregate as well. There is also a Google Maps + Microsoft Virtual Earth browser that I mentioned previously called FlashEarth.
Technorati Tags: [Yahoo! Maps], [Virtual Earth], [AJAX], [Flash], [Google Local], [Google Maps], [GoogleMapsMania]