Here is a list of 2 letter domain name extensions available for registration on Micro Domains.
Each 2 letter TLD has an associated country. For example, .mx domains are associated with the country of Mexico. Similarly, Colombia is the entity for .co domains.
While far more 2 letter TLDs exist than this, all TLDs on Micro Domains are available for registration without restriction.
A domain name with a 2 letter extension allows you to add 1, 2, or 3 letters to the left of the "dot" and still have a short domain name of less than 5 letters long.