Someone figured out a way to merge Terraserver data with Google Maps. The best bit, though, is that you can get a Firefox plugin called Greasemonkey and a DHTML script to make it happen in your own browser:
Someone’s hacked a totally custom map into Google (Chicago Transit Authority maps) and he’s used a Greasemonkey script to integrate it directly into the regular maps.google.com map. If you’ve got Greasemonkey installed (on Mozilla), then right-click on this script: topo_monkey.user.js, and select “Install user script”.
Then, reload Google Maps, and you should see some new options (Terra, Topo, and Urban) appear next to Map and Satellite.
Why does this kick ass? Because Terraserver did not have as cool of interface as Google Maps for scrolling/zooming/browsing the maps. And now it does with this little Greasemonkey script! But I predict that Microsoft will shut this interoperable capability down somehow since all the eyeballs will be looking at Google Maps instead of Terraserver.