Geonames reverse address geocoding
What is that? You know about typing an address and having Google Maps show you where the address is? Well, the reverse of that is looking at a Google Map and having a program tell you the street address. This Google Maps mashup allows you to navigate around a map and when you have found the place you want, click on the map and it will tell you the street address that corresponds to the place -- or at least one street address since many times there are multiple ways of looking at a point on a map.
Reverse address geocoding is just one of the many things done at www.geonames.org.