NAntGraph itself is licensed under the GPL v3.
Note that the GPL does not apply to the output generated by NAntGraph - you are free to use NAntGraph to generate documentation for commercial projects without being influenced by the GPL.
For more information on the GPL v3, see http://www.gnu.org/copyleft/gpl.html
Update: As NAntGraph2 is no longer dependent on Microsoft GLEE, licensing is much simpler - only the GPL applies.
