As CC licenses doesn't need giving back the source and GPL is too complicated with the "warranty" stuff. I try to formulate my own license here:

1. You are allowed to use the unmodified NewGRFs for your needs as you like. (commercial and whatever...)
2. You are allowed to use the source and modify it as long as your work will go back to the community. With following points.
   1. free and opensource
   2. credits also for co-authors

Thanks you very much for the fish. ;-)

Amden, 29.12.08 - Marcel "Ammler" Gmür