One of the most exciting possibilities and one that is already being realized to some extent on the Web is rich collaborations among authors in different places, in different fields, and even from different cultures -- authors who may not even know each other.
In the example below from a module on divergence, I used a parser written by Darius Bacon that he has made freely available. See his file on copying.
I've used his parser in several different applets including the one below. They, along with my other applets, are also available for use by other authors, either as they stand or as part of other works. The only restriction is that the chain of authors must be acknowledged and further use can not be restricted in any way. This applet illustrates the way that the density of a migrating population, for example, changes as described by the divergence of the vector field describing migration.