VoxBo is a highly modular and interoperable collection of free tools for brain image manipulation and analysis, focusing on fMRI and lesion analysis. VoxBo can be used independently or in conjunction with other packages.
The GNU General Public License; see: http://www.gnu.org/copyleft/gpl.html
The GPL requires that source code to a tool or resource be made available, and that modified versions of the tool or resource (including new tools which include part of the original tool's source) make the modified source available to users.
A developer of a tool or resource distributed under the GPL ensures that users of the tool and of modified versions of the tool may always inspect the source code and modify the source to suit their needs, and that users may modify the tool as they see fit without being obliged to distribute their changes. While this may seem to encourage multiple versions of the same tool, the result is often that changes are passed back to the tool originator and incorporated into the original tool.