Linus Torvalds first released the Linux Kernel in September of 1991 under a very restrictive license requiring that the source code must always be available, and that no money could ever be made off of it. A few months later, he switched to the GPL, or GNU General Public License, the license that has been used for the Linux kernel source code ever since. A recent thread on the lkml discussed some of the gray areas of legality where it’s not explicitly clear what the GPL allows.
Link: KernelTrap.org
