Eros Os is an Operating System providing the following:
Pervasive threading
Some of the issues that the Eros Os addresses, especially the Capability System are also talked about in Os Wiki, Wiki Os, No More Databases and Killer Operating System.
What Eros Os does not provide:
a modern, high-level Capability Security Model -- Orthogonal Security
versioning, not even at a coarse grain
state of the art design, or even
EROS, boldly going where the Cambridge CAP Computer went thirty years before.
Anyone care to argue for this quip?
Anyone care to dispute it? This is one of those things that are dear to support, easy to counter. So if it CAN be countered ....
Well, Cambridge CAP Computer used hardware enforced capabilities while Eros uses the usual priviledged/proplem mode and memory mapping kernel architecture to implement them. As far as I can make out in chapter 5 in www.cs.washington.edu 
Cambridge CAP Computer uses capabilities only as protected memory segment references and doesnt allow for user implemented objects. Objects composed of natively provided objects by the Eros Kernel. So I would say Eros, and its predecessor KeyKOS went a lot further than CAP and didnt fall into the same quackmires. --Zarutian
See www.eros-os.org for further details. The docs and faq are good. --Dennis Decker Jensen
From that site:
"The current EROS system is still not a complete, runnable system. We have a group at Hopkins that hopes to cross this hurdle this summer. For now, we encourage you to download only if you have a research interest in running the code, you are a potential EROS developer, or you have some desire to read other people's kernel source code. Note in particular that while the system image that lives in eros/src/base/sysimage/ now builds, it still doesn't do anything useful at this time."
As of 2003-03-04 this project appears to be dead.
EROS is not dead, although it has been progressing rather slowly. (I think this is partly due to the developers implementing and transferring EROS to their own source code management system, OpenCM <www.opencm.org >, which has been a bit of a diversion from working on the OS.)
As of 2005-10-20 the web site indicates that work on Eros Os itself has ended; its continuation is Coyot Os.
It is also being continued as Capr Os --Zarutian
My Operating System, OIX, is based upon technology similar to this -- Capability Systems and Orthogonal Persistence (or at least the persistence part ;). I am not sure about threading as of now [01Nov] -- Be Os is well off with it, but I am a Unix (translation: Linux) guy, so I'm not really too into threads. Also, I have not done too much thinking as to the type of kernel. The last thing I will say is that OIX has an Executable Hierarchy influenced by the Java Language, the Smalltalk Language and the Oberon Operating System. -- Aluo Nowu
See original on c2.com