GeoNames.org - provides geocoding services. GeoNames provide a huge databases of place names (with coordinates) organized in a hierarchy (eg. country/state/region/province/etc...). Also there is a query that allows to retrieve the geo tags within a range from a specific point. Geonames is integrating geographical data such as names of places in various languages,elevation, population, alternate names, Administrator division type,continent, Address,timezone & weather observation.

Geonames provides RESTful based webservices with both XML & JSON rendering. These seems to defined by seasoned developer & have Style enum (Short,Medium,Long, Full) for defining result verbosity & many such developer friendly criteria APIs.

For off-line applications, geographical database is available for download free of charge under a creative commons distrubution license & can leverage the service by downloading all the data. Geonames.org claims that it has already serving up to over 3 million web service requests per day & has the ability scale.

I guess applications (especially free & open source ones) have excellent opportunity to integrate with this service. Web applications can be relieved from maintaining the huge database of geo services. They can just store the geo-ids & can rely geonames to fetch the details. Hierarchical data is always to difficult store & fetch in standard RDBMS & now there is no need to maintain this data. GeoNames includes the reference implementation in all popular languages. I just checked the java implementation & it was not that good. It could be have been made more easier (or fluent).

The important point to note is that high-profile users of GeoNames include Slide.com & LinkedIn.

Reference
http://www.geonames.org/export/ws-overview.html
http://www.geonames.org/export/place-hierarchy.html
http://en.wikipedia.org/wiki/GeoNames