PSA: All my RiveScript modules are now released under the MIT License instead of the GNU General Public License.
It basically means you’re free to use the modules in a closed source application without needing to release your entire app as open source under the GPL, which is what the GPL license required. Examples of other libraries similarly released under the MIT License are jQuery, Ruby on Rails, and CoffeeScript.
Versions where the license change happened:
Perl - v1.36
Python - v1.06
Java - v0.03
JavaScript - v1.0.4