Geocode

A geocode is a geographical code to identify a point or area at the surface of the earth.

The code systems can be described by:

how the area is coded (number, letter, mixture of both, other)
which part of the earth is covered (whole earth, land, water, a continent, a country)
what kind of area or location is coded (country, county, airport, railstation, city, ...)
whether an area or a point is coded
time for which the code is valid

List of some code systems

Some of these code systems are free for use, others have different licences.

Car License plates
C-squares - compact encoding of geographic coordinate bounds (latitude-longitude)
FIPS country codes (FIPS 10-4), area code, administrative, free
FIPS place codes (FIPS 55) U.S. only, free
FIPS county codes (FIPS 6-4) US only, free
FIPS state codes (FIPS 5-2) US only, free
Georef, a military / air naviation co-ordinate system for point and area identification
IATA airport codes, area /point codes, airports
ICAO airport codes, area /point codes, airports
IANA country codes similar to ISO 3166-1 alpha-2
IOC country codes, area, worldwide
ISO 3166 country and subdivision codes
ITU-R country codes
ITU-T country calling codes
ITU-T mobile calling codes
Maidenhead Locator System
MapDot Protocol: world locations coded into a zone sequence[1], free
MARC country codes
Marsden Squares
NAC, area codes (area can be indefinitely small)
NUTS area code, partially administrative, worldwide: countries, europe : country to community
ONS code, UK only, administrative
Postal codes, area, worldwide, country-codes by UPU, free
Quarter Degree Grid Cells
UN M.49 region codes, area code, continents, countries (like ISO 3166-1 numeric)
SALB (Second Administrative Level Boundaries), by UN [2]
UN/LOCODE, area, administrative, cities
UTM
WMO squares