Unless I’m mistaken (and I may be), there are three major types of Unix-like kernels you can download and use for free right now. There’s GNU/Linux, BSD and one you probably haven’t heard of, GNU Hurd.
What’s the Hurd? According to its web site, it’s described as "…the GNU project’s replacement for the Unix kernel. It is a collection of servers that run on the Mach microkernel to implement file systems, network protocols, file access control, and other features that are implemented by the Unix kernel or similar kernels (such as Linux)."
Should you use the Hurd? Not yet. You could, but I wouldn’t recommend it because there’s no graphical environment for it, Java is not available for it yet and it has a ways to go before it’s ready for the masses.
You can however get a very detailed report (as much as could be tested for now) on how Hurd performs right here, where it’s stated there are plans for a release of Debian "Wheezy" that will use the Hurd kernel in late 2012 or early 2013 – maybe. An exact release date isn’t known at this point.
Why use an OS that uses the Hurd at all? This is best explained in the Advantages web page of the GNU Hurd web site, specifically:
Unlike other popular kernel software, the Hurd has an object-oriented structure that allows it to evolve without compromising its design. This structure will help the Hurd undergo major redesign and modifications without having to be entirely rewritten.
One advantage of the Hurd’s separation of kernel-like functionality into separate components (servers) is that these can be constructed using different programming languages — a feature that is not easily possible in a monolithic kernel.
It is possible to develop and test new Hurd kernel components without rebooting the machine.
There are legitimate reasons for the Hurd to exist. Good reasons. It will make it easier for developers to develop on and provide convenience whereas you don’t have to reboot as often. For you UNIX and Linux fans out there, GNU Hurd is one to watch as it should be quite good for future computing.