Wikipedia:

G13 is the alleged codename for a strain of marijuana, supposedly developed by several U.S. government agencies during the 1960's. According to myth, the CIA, FBI, and other agencies procured the best strains of marijuana from breeders all over the world.

At a secret installation in Mississippi, they bred many new hybrids. Putting these together, they created a strain that was more powerfully intoxicating than any of the original hybrids.

This strain, G13, was a primarily indica phenotype strain and provided a very potent smoke and a completely mellow high.

Allegedly a single cutting of this potent strain was stolen by an employee at this government facility and released into the public domain. This is of course a myth used by dealers to sell product, or seed banks to sell seeds. Some modern breeders claim to have strains which are descendents of the original G13 strain. While this is most likely not true, chances are the strain they are selling is much more potent than anything produced 30 years ago.

To date, no government agency or independent verification of this strain has been released. Other rumors say G13 was developed by the University of Washington's cannabis-testing-facility in the 1970's for medical testing, but this, too, has not been quantified.