What license to pick...

[Forrest J. Cavalier III]

It is nice to see someone ask questions before they take a license and
assume it does something it does not, and mis-use it.

> Does the GPL allow us (the toolkit creators) to ask a fee for commercial use
> of our toolkit?

That is a question that can be interpreted in a few different ways
which change the answer.

You, as the toolkit creators, can distribute the software for a fee 
as you choose.  You can even decide to distribute by charging only 
certain customers, and not others.  

A Free software license (...Free with a capital F here refers
to freedom: the abilities of anyone possessing a legal copy of
the source code...) means the authors have given freedom to use
and distribute the source code.  That freedom includes permission
to redistribute under the license, with or without charging.

Each Free software license is specific on what is and is not
included in the freedoms it gives.  It is hard to summarize
the differences, you should read the licenses.

You mentioned you were inclined to use the GPL.  Under the GPL,
you are prohibited from making additional use restrictions.
      - You may not require royalty payment for commercial (or any) use.
        (A royalty refers to a payment to the author when a copy
        is used.  This is different than charging for transferring
        a copy.)

      - You may not require someone pay you when they transfer
        or use a copy.  

      - You may not limit the fees someone may charge for
        making a copy.

      - You may not require that modifications be sent to you.   
        (But the GPL does require that when a derivative work
        is distributed, the GPL'ed source code is included.)

Other Free licenses have different requirements and restrictions.
But to be a Free license, they may not prohibit use in a commercial,
or certain environment.  (That's what the non-discrimination
clause of the Open Source Definition says.)

When/if you start accepting modifications from the community,
then you have to be careful that the contributions can be
incorporated and licensed the same way.

Everyone here strongly suggests you use an existing, approved
license instead of writing your own.

The difficult "leap of faith" for authors is to believe that
the marketplace by nature (rather than through closed-license
requirements) generally preserves the value in software.

As long as your software fits into one of the business cases
that Eric Raymond discussed for Open Source software, then
you'll probably be happy using an Free/Open Source license.  

And if you don't match one of those business cases, then,
yes, you will be giving away value.  The open source license
will ensure the value is preserved or increased (more people
will benefit) but if you can't stay in business, then don't
do it.

Forrest J. Cavalier III, Mib Software  Voice 570-992-8824 
   http://www.rocketaware.com/ has over 30,000 links to 
source, libraries, functions, applications, and documentation.