An Offline Reverse Geocoding Java library
Uses KD-Trees for extremely fast placename lookups
Licensed under LGPL 2.1
A C# port by Necrolis is available at https://github.com/Necrolis/GeoSharp
Usage:
First download a placenames file from http://download.geonames.org/export/dump/
Allcountries.zip from that site is comprehensive however if you're on mobile try the cities1000.zip file. It's 1/80th of the size.
Then simply
ReverseGeoCode reverseGeoCode = new ReverseGeoCode(new FileInputStream("c:\AU.txt"), true);
System.out.println("Nearest to -23.456, 123.456 is " + geocode.nearestPlace(-23.456, 123.456););