A while back I created an application called YPlaces. As a tool for exploring the GeoPlanet API it proved quite enlightening.
My latest mashup is called SakMap. It has a lot of things going on but my favorite feature, and one perhaps of interest to this group, is the GeoPlanet query function. The current center point of the map is reverse-geocoded and the resultant U.S. city name is passed to GeoPlanet for it's Children WOEID's. These become links and their exploratory properties are exposed.
A quick way to try this out is to enter a city name and state in the "Find Place" field and then click the Tree icon to locate Children for that city. Click the individual Children items to center the map to that place, and explore from there.