See also 3 character ISO Countries and Territory codes

Sources:

Source	Number of political entities	Other/Comments
ISO 3166 Country Codes	240	The International Organization for Standardization (ISO) assigns official country names and codes [See: ISO 3166-1 Alpha-2 Code lists] (Codes for the representation of names of countries and their subdivisions) (e.g. us, ca). They also assign numeric and 3 character codes; see Country and Territory List for other codes. Country Name List The Maintenance Agency (3166/MA) consists of National Standards organizations, the UN, ITU (see below), Internet Corporation for Assigned Names and Numbers (ICANN) and others. ISO 3166-1 Country Codes: Alpha-2: ZA Alpha-3: ZAF Numeric-3: 710 English short name: SOUTH AFRICA English full name: Republic of South AfricaPrince-Édouard English remark: Includes Marion Island, Prince Edward Island French short name: AFRIQUE DU SUD French full name: République sud-africaine French remark: Y compris: l'Île Marion, l'Île du Prince-Édouard Alpha-2 Code lists with short names are avaliable on their web site. There is a fee for the Alpha-3, numeric and "full name" codes. ISO 3166-2: Country Subdivision Code (e.g. US-CA California), ISO 3166-3: Codes for Formerly Used Names of Countries List with Full Name at
Telephone Country Codes	230	Numeric Telephone Country codes are assigned by the International Telecommunication Union (ITU). Places like the Palestinian Territories, British Indian Ocean Territory, South Sandwich Islands have ISO codes but no Telephone CC. In addition there are 12 special codes for things like, Inmarsat (Pacific Ocean, Atlantic, ...), International Freephone Service, etc..
Top Level Domains	243	Internet Top Level Domains (TLD) are assigned by the Internet Assigned Numbers Authority (IANA) and are generally the same as the ISO 2 digit country code elements (‡ except for the United Kingdom). In addition TLD's include .com, .org, .gov, ...

Updates:
July 2003 Yugoslavia (YU) replaced with SERBIA AND MONTENEGRO (CS)
May 2002 The Alpha-2 and 3 codes for East Timor were changed from TP and TMP to (Timor-Leste) TS and TLS.

The "+" in front of the country code is the international telephone format notation for country code. TLD is the Top Level Domain for Internet addresses (URLs) which is also the ISO 2 digit Country Code

See Also: Country and Territory List
Telephone Information for Area Codes and City Codes
ISO Codes - An Internationl Dial Code Directory
Internet Domains
TLD Country Codes at uci.edu
ISO, Windows and Macintosh name/code list at unicode.org The United Nations also publishes a 1-3 character country code for distinguishing vehicles in international traffic.
The ITU also has a 1-3 character code for member nations
US Patent Office Country Codes. See links at ISO for others.