Apartments listed on Google Maps
People have started annotating Google Maps satellite images - sadly not on Google Maps itself, but rather on Flickr using exported images.
myGmaps provides a direct Google Maps annotation system. I can't imagine this service will not be shut down as they are effectively stripping all Google content except the map itself.
Also, people are sightseeing on Google Maps.
GMail is being used as an online back system.
Greasemonkey simplifies and automates end-user customizations of websites (for Mozilla users only). This is a good thing as we recall from the allmusic redesign scandal.
Yahoo's term extraction service has a nice interplay with Technorati.

Embracing user defined extensions of your service can be a powerful thing...