Hello!

I am going to develop a CMS like PostNuke, Geeklog, etc. The CMSs above are released under GPL and force custom developed modules (for example hotel booking module) to be GPL too.

I want to distribute my CMS free and open source, but want let third party developers to develop and SELL their own modules.

What license is the best choice for this model? LGPL, BSD, other?

Thank you for your help!