On July 19, 1999, Fourth World migrated its line of SuperCard development tools to the Open Source distribution system, in which both the tools and the source code from which they were made are all freely available subject to the terms of the Fourth World Public License.
To learn more, check out these links:
The Official Home of the Fourth World Open Source Alliance
The Fourth World Public License
Press Release for Fourth World Open Source Alliance
The Fourth World Open Source Alliance FAQ