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 you 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 proper credits. (also of co-authors)

Thanks you very much for the fish. ;-)

Amden, 29.12.08 - Marcel "Ammler" Gmür