The Voice User Interface Designers Toolkit is an open-source project that supports designing and maintaining a text or voice user interface.
The code was placed into the public domain in 2007. Since then, a number of organizations have adopted the toolkit for their own use. The code is free, as in free beer, and licensed under the GNU General Public License version 3. If you would like support, or would like to schedule a training session, please contact Phil Shinn at phil.shinn@gmail.com.
The most recent version is VTK_2016_06_05. The most recent version uses Chrome for the GUI. If you're looking for the older Visio version of the tool kit, click on the versioning indicator 'v.2' and select v.1.