Mithrandir is right. Since the modules are considered part of the whole application (since they will not function without the core which is released under the GPL), they are automatically GPL, or similarly licenced. This means that you have the right to the source code, if you get a legally obtained copy. Also, you have the right to edit and redistribute the code. Even when you have paid for the module in the first place. To prove this point, show me a place where you have to pay for the code of a GPL-ed application (NOT services like packaging, documentation, etc.). There aren't any, simply because of the GPL. This doesn't mean you can't get paid to develop, write, create, design or customise a module or site powered by XOOPS, all it means is that you can't limit the use of the code other then what the GPL states.
This has actually been submitted to an attorney and checked.

As for the idea about the credits page, most modules have this in the code itself (the credits). It is our intention to add a credits page to each module admin, as well as to the XOOPS core. Adding it to the page source can be a good idea, but it may not be the best. (it would require the addition of a smarty variable to the theme.html, which is assigned by each and every module). Then again, I don't know a better way to do this (although it is very hidden).

Herko

Ok, as said, I'm not an expert. But somehow it still seems a little odd. How is developing and then selling a license of a module (and not allowing the buyers to edit and redistribute the code) different from developing a program for Linux? Or for that matter a entire distro like Redhat enterprise edition. It isn't legal to buy the enterprise edition and then distribute it for free. It kind off seems like the same thing, yet one is allowed and the other isn't.

It's not that I intend to sell a module or something, but I'm interested in how this works. I'll take a look at the license.

Isn't that because RedHat Enterprise is not GPL'ed?

I must admit that I am not all that into how Linux, Linux programs and Linux distros are licensed, so someone else may have to add that bit.

RedHat Enterprise is indeed not GPL. But it uses code and entire programs that are GPL. The GPL states that you can ask money for services concerning the program (transport, bundeling programs, etc), not for the products itself. It seems that when you write your own code/program that only works when a GPL program/OS is being used for it, it can be sold under a different license.

I do know that a number of laywers have actually been arguing about this licende the past year. So if they don't understand, how the hell am I suppost to anyway .

Thanks for all the responses.. I just wanted to know if this was "ok" so I wouldn't be accused of trying to steal someone else's code..

I don't want to remove the credits from the modules like Agenda-X which shows

(Agenda-X v2.0FINAL by wjue(Wang Jue) & guanxiCRM.com) at the bottom for any malicious reasons, but more for my feeble atempts to design the page to look better.

Besides, I don't see how that would be remotely possible because I don't have the php, MySQL skills to create something this cool.