The Linux Foundation has released 2012 Linux White Paper which analyzes developers and contributors of the Linux kernel from 2.6.36 to 3.2.The top ten contributors are: Red Hat, Intel, Novell, IBM, Texas Instruments, Broadcom, Nokia, Samsung, Oracle, and Google. The software giant Microsoft's contribution ranked at 17, while the company's CEO Steve Ballmer has claimed previously that Linux is a cancer. Microsoft engineers have contributed 688 patches, which are mostly related to Hyper-V virtual machine technology. The top 10 developers from Microsoft are: Thomas Gleixner, Joe Perches, Chris Wilson, David S. Miller, Axel Lin, Eric Dumazet, KY Srinivasan,, Johannes Berg, Al Viro and Tejun Heo.

But some people think that Microsoft only changes the Hyper-V which is their own product. they
have only limited contribution to Linux. Also, most of the modifications are  just a few lines but with many submissions.