AtheOS is a free alternative operating system that runs on common X86 hardware and even supports SMP. AtheOS is a bit further along in the development cycle than most other fringe operating systems and ports of many common software packages are available. Including Apache, PHP3, Emacs, and BASH.
AtheOS was designed to be more of a desktop operating system than anything else. It is not simply a Unix clone. Many of the common Unix tools will compile on AtheOS though, because it is largely Posix compliant. The AtheOS GUI is integrated into the system, and works similarly to X Windows, but it is much must faster. Like BeOS, AtheOS goes straight to the GUI, there is no "command line only" mode.
The AtheOS kernel is brand new and is not based upon any other pre-existing code. SMP support and TCP/IP networking are built into the kernel itself, for fast performance. Device drivers and file systems are loadable and processes can communicate with each other in several different ways.
Many people seem to think that AtheOs was inspired by BeOS. This is not the case. It has actually been in development since before the first BeBox was ever built. Here is what the developer of AtheOs had to say about BeOS.
I started working with AtheOS before the first BeBox was built and I had a fully working object oriented message based GUI up and running long before I saw BeOS for the first time. Back then it didn't have an "application-server" since it was a single-address-space OS and there was no need for a central GUI server. Later when I implemented memory protection I moved the window-handling into a central server. This is the same approch as both BeOS and X11 use but when the alternative is to move the entire GUI into the kernel it is a pretty obvious choice.
Cloning BeOS has never been a goal. I have looked at BeOS and other OS's to see how things are done to avoid reinventing too many wheels and since BeOS is a very nice desktop OS many of the GUI design choices has been affected by what I have seen in BeOS. If you look at the class-hierarchy you will find many similar classes and the event system based on the BMessage-like container class, which is absolutely inspired by BeOS. If you look closer at the implementation of everything (or try port over an application) I believe you will find that BeOS and AtheOS are not as similar as they first appear.