To quote the Wikipedia:
Geohash is a latitude/longitude geocode system invented by Gustavo Niemeyer when writing the web service at geohash.org, and put into the public domain. Geohashes offer properties like arbitrary precision and the possibility of gradually removing characters from the end of the code to reduce its size (and gradually lose precision).
Geohash is also the name of a Common-Lisp that implements the algorithm. For example:
> (encode-lat/lon-to-string 42.584 -5.59 5) "ezs42" > (decode-string-to-lat/lon "ezs42") (42.60498 . -5.6030273)
The code hasn't been optimized but it passes the few tests I've thrown at it. Suggestions welcome.
A darcs repository is available. The command to get it is:
darcs get http://common-lisp.net/project/geohash/
Geohash is also ASDF installable. Its CLiki home is right where you'd expect.
There's also a handy gzipped tar file.