Proper license for plugins

[Kelly Anderson]

It would seem from my limited research that the GPL is the most common 
license used for open source projects. My (primitive) understanding of GPL 
is that if you improve code under the GPL, you must release or offer to 
release the source code to the improvement. Fair enough.

Obviously, if you write a program on the Linux platform, you are not 
required to release the source code under the GPL just because it runs on 
Linux. You could actually sell such a program, right? I assume people are 
actually doing this.

I would guess, but am not sure, that if you created a plugin for Emacs, you 
would not be required to release the source code. I do not know if you 
would be able to sell such a plugin.

If you were thinking about creating a core environment that could be 
extended through a plug-in architecture (similar to how Emacs works) and 
you wanted the plug-ins to be able to be either open source OR proprietary, 
would the GPL be a reasonable choice or is there a better license out there 
for this purpose?

-Kelly