In its newest annual report, the Linux Foundation has announced that, for the first time, Microsoft is one of the 20 largest contributors to the Linux kernel. The company added one percent of the kernel's new code between 2010's 2.6.36 release and the 3.2 version released in January (the newest stable version is 3.3.1) — quite a stark contrast from 2001, when CEO Steve Ballmer famously referred to the open source project as "a cancer."
The biggest contribution coming from Microsoft was the cleanup of the company's Hyper-V drivers, which it originally open sourced in 2009. This wasn't an altruistic move, however — the drivers, which improve the Linux virtualization experience on Microsoft's Server products, were only contributed after Stephen Hemminger, a principal engineer with Vyatta, noticed they contained both open source GPL (GNU Public License) components and Microsoft's own code. Since the GPL doesn't permit mixing of open and closed source in publicly-released projects, Microsoft was believed to be in violation of the license, and released the drivers to the open source community shortly after the discrepancy was spotted.
The code sat in the Linux kernel staging area — where standalone drivers and filesystems go when they're not yet ready to be added to the main kernel tree — for nearly two years before Microsoft did the cleanup needed to get them added to the mainline kernel, most of that work falling to K.Y. Srinivasan, who single-handedly changed 9709 lines of code (1.1 percent of the total) for the 3.0 kernel release in July. Now that the transition from staging area to mainline kernel is complete, we're expecting a much smaller contribution from Microsoft next year.