BitKeeper was the distributed version-control system that Linus Torvalds and the other kernel developers were using over a decade ago. It was a proprietary product, but the Linux folk were allowed use of it for no charge, under a licence that prohibited reverse-engineering, and that obligated them to uprade to newer versions with whatever new licence terms the company wanted to impose. It was a stoush over this reverse-engineering prohibition that led to BitMover giving notice it was cancelling this agreement, which in turn led to Torvalds creating Git. And the rest, as they say, is history. <http://phoronix.com/scan.php?page=news_item&px=BitKeeper-Open-Source>