The Linux kernel is an open source, Unix-like operating system kernel. It is the core of the Linux operating system, responsible for resource allocation, low-level hardware interfaces, security, simple communications, and system call functions. The Linux kernel is distributed under the GNU General Public License (GPL) and is developed by a community of thousands of volunteer developers around the world. The Linux kernel is capable of running on a variety of computer architectures, from small embedded systems to mainframes. It is the most popular operating system kernel in the world, powering millions of computers and mobile phones. The Linux kernel is a stable and reliable platform for running web servers, databases, and other applications.
FreeBSD
GNU Hurd
Linux-libre
Redox
The GNU Hurd kernel is not compatible in any way with anything compiled for the Linux kernel. The system libraries are different, due to different design decisions made when making the two kernels.