Would it be possible to write an OS that could run mac, linux, and windows stuff all together?

I highly doubt it, Windows, Mac and Linux run on different kernels have programs written completely differently. You can run most Windows programs on linux in WINE, but I don't think there's something similar for Mac.

You could run some Windows programs on OS X with Darwine but I don't think there is anything to run Linux programs on Windows or Mac.

There is an ambitious project called E\OS (http://meos.sourceforge.net/) that apparently can run Mac, Windows, Linux, AND BeOS programs (all I care is the windows and beos compatability). I have no clue how active it is, but they seem to have a lot of BeOS documentation and that's about it... there are some screenshots but they are mostly of BeOS programs.

There's LINE for running Linux programs on Windows, but it's highly incomplete. Also, since most Linux software is open source, it can be (usually with little to no code modifications) compiled on Mac OS X (directly) and on Windows (using Cygwin) , but you'll still need X11.app on OSX and Cygwin/X on Windows for graphical applications.

The SoftPear project was an attempt to modify FreeBSD to run Darwin/x86 binaries, which later would have allowed (by copying the required libraries from Mac OS X) running Mac OS X binaries on FreeBSD. I don't know how far they got.

FreeBSD can already run most Linux binaries (as, aside from a small number of Linux-specific calls, it's only a different system call calling convention). WINE runs on FreeBSD, and if SoftPear will ever get anywhere, FreeBSD will be your platform of choice.

I heard of that project, but its not an os. its an emulator. A lousy looking one at that.

I second that. Either FreeBSD or Mac OS X (with X11 and some modifications to run Linux apps, and with Darwine to run Win32 binaries).

HHHmmm, could be interesting

It would be an abomination in the eyes of God.

If u got the working parts together, it would be rather *easy* to put it together since Mac OS X is able to install using custom packages ( just take a look at the JaS 10.4.8 Intel/AMD SSE3/SSE2 release available on almost any torrent site) So I would definately go for OS X Tiger, dunno bout leopard when it comes to compatibility

Windows Vista can run programs written for DOS, and for Windows 3.0, and also, as proved by Nathan Lineback, you only have to change 1 bit in a program to make a Windows 1 program usable with Vista. Daft as it might sound, the systems' APIs aren't as similar as they appear; they run (as most people here will know already) through the NT Virtual DOS Machine, which emulates the API environment for which said program was written. So there should be a way of writing an OS that emulates the APIs for all of these systems that have been mentioned, shouldn't there. I know it would be stupidly difficult to do, but I think it must be do-able in theory.

Obviously, it's not going to be as simple for different processor architectures...