Although much of the software discussed has been developed with public funding, none of it is open source, which allows anybody access to the underlying software code. The beauty of open source is that users can customize the code to their own needs, find errors, or even offer improvements. Most open-source licenses enforce this spirit of collaboration by requiring any redistributions of the code to also be freely available. Some notable examples of open-source software include Firefox (browser), Linux (operating system) and, lately, Android (mobile phone operating system).
What does this mean in terms of modeling software? Without open-source capabilities, it means that the code is largely hidden and customizing the software is difficult, if not impossible.