The source code is distributed under the terms of the GNU General Public Licence. For more information, refere to this page: http://www.gnu.org/copyleft/gpl.html.
Documentation licence
The Header Browser's documentation is placed under the GNU Free Documentation Licence. For more information, see the page: http://www.gnu.org/copyleft/fdl.html.
Askings
If you want to modify the Header Browser's source code or documentation, I'll just ask you to send me the modifications, and perhaps discuss their utility. I think it is useless for coders to have multiple versions of a tool like Header Browser.