These days, when one talks about free software, the first word that comes to mind is Linux—be it the kernel or a distribution based on it (which would then be a GNU/Linux operating system, and its flavour marked by a brand name: Red Hat, SuSE, Mandriva, Debian, Ubuntu, Slackware...)
At one time, there was another project worthy of note: BeOS. It wasn’t POSIX-compatible, but it was neat. But now, only free *NIX prevail... really?
Another viable free software operating system—help wanted
ReactOS is an old project. It actually started around 1996, but started producing code only recently. It aims at implementing all of win32 according to specs, be it hardware or software.
Yes folks, it means that ReactOS would basically run all of Microsoft Windows’ drivers and applications natively.