MetExploreViz provides an open source licensing, for developers that are willing to release the source code to their applications.
To use this license you must follow the terms of the GPL v3, highlights below.
GNU General Public License version 3 (GPL v3) Terms & Conditions
You can elect to use the GPL v3 license without paying a fee, provided you must comply with GPLv3 license terms.
If you create software that uses GPL, you must license that software under GPL v3 (see GPL FAQ), and;
If you create software that uses GPL, you must release your source code (see GPL FAQ).