Remember the 8-bit version of Maps that Google "announced" for the NES this weekend? April Fools' Day is over, but Duncan Barclay's MapsTD has put it to use by adding a simple tower defense mechanism that can be played anywhere on (or even outside) Earth. After you enter a location, MapsTD will spawn a path to defend and give you £200 to spend on towers with simple range, speed, and damage attributes.
It's not a particularly complicated game, and we haven't yet found much of the enemy and tower variance that's available on other web defense games. The conceit, though, is pretty clever, and it uses the various Maps overlays to great effect. By default, it uses the 8-bit overlay, but you can switch it to play in satellite view or even on the Moon. It's already one of the more fun uses of the Google Maps API, but it would be great to see it updated to keep up with Google's changes. We haven't yet seen a tower defense game, for instance, where it's possible for enemies to get stuck in traffic during rush hour.