GMT = Generic Mapping Tools. A set of tools for manipulating geographic and Cartesian data sets.
Installed versions: 4.5.7
The best reference to GMT is an article by Wessel and Smith published in EOS, the transactions of the American Geophysical Union, on Oct 8th 1991 (Wessel, P. and W. H. F. Smith, Free software helps map and display data, EOS Trans. AGU, 72, 441, 1991).
Licensing: GNU GPL