Oberon is an Operating System developed by Niklaus Wirth and Juerg Gutknecht. See also Oberon Language.
Some of its cool features are
Tiling windowing system. Windows are arranged in two tracks. You never have a mess on the screen and windows are always rectangular. I'd really like to have a window manager like this! (a Window Manager That Actually Manages Windows? Try www.nongnu.org 
or modeemi.cs.tut.fi )
Integrated Text format with embedded objects. You can middle-click on a Module.Function name and it automatically loads the module and calls the function.
More information can be found at www.ocp.inf.ethz.ch older information is available here www-old.oberon.ethz.ch and there bluebottle.ethz.ch . For a description, see www-old.oberon.ethz.ch .
If I remember correctly, one of the design requirements for Oberon was that Oberon should compile Oberon within less than a minute. Apparently, the motivation for Oberon was that Niklaus Wirth had worked with Cedar (a fine tool) at Xerox Parc and felt that it was just too large, so he went home and rewrote it.
I've found it short on documentation to help me get started. After 3 hours I gave up.
It uses UI metaphors and conventions that are wildly different from the modern "standard", which can make it rather challenging to learn about. But this also makes it a treasure if you're curious about how things can be done differently from the modern "standard", and it does actually have useful applications made for it. Curious. UI and app examples?
Useful apps: Windowing system, compiler, text editor, web browser and server, at least. Interesting UI ideas: It uses extensive mouse "interclicking" or "chording", clicking one mouse button while holding another (Ctrl-* or Alt-* keyboard shortcuts are examples of keyboard chording, Oberon does it with the mouse too). Tiled window system. Exposure of the system API to the user as commands. Probably more I don't remember; one can download a version that will run on top of Linux or Windows, and (as far as I recall) it comes with a decent assortment of introductory documentation. -- Simon Heath
Oberon Influences, Conspiracy Theories
I recently found out that Oberon has a strong influence on some other products including the Plan Nine editor called Acme (which I think can run on other operating systems).
Quote:
"Acme is a text editor and graphical shell from the Plan 9 from Bell Labs operating system, designed and implemented by Rob Pike. It can use the sam command language. The design of the interface was influenced by Oberon. It is different from other editing environments in that it acts as a 9P server. A distinctive element of the user interface is mouse chording." --en.wikipedia.org
One of my conspiracy theories, is that Rob Pike (and other Unix programmers) are very much in love with Wirth and his student's works from Switzerland. For example consider Limbo Language, Sawzall Language, Inferno, Acme Editor, and several other heavily oberon/modula/pascal influenced products. They are very very very similar in syntax and ideas. They've shown several obvious similarities - striking similarities actually.
Consider also the C programming language, which is very much like an extended version of Wirth's original Pascal when Pascal first came out with its shortcomings. C was simply disguised in completely different syntax and more flexible than Pascal - and now Limbo and Sawzall and other languages are looking more and more like Wirth's ideas. This is a good thing, not a bad thing - of course - it's perfectly okay for ideas to be shared. I think more credit should be given and less hatred, though, toward Wirth. --Conspiracy Theorist
Even consider Rob Pike's paper here, which again has a lot of Wirth similarities in the Type system declarations:
And consider this paper which mentions Oberon numerous times (Plan Nine's help system starting phases):
So, it is not such a Conspiracy Theory - but rather a fact that Oberon is having an influence on famous Cee programmers who'd we would have otherwise falsely assumed hated Wirth.
See original on c2.com