"The question of whether proprietary closed source code is
present inside open source code is a huge issue. It sits at the
crux of the SCO battle against Linux and it is also at the center
of another controversy surrounding an open source effort to create
a free clone of Microsoft's Windows operating system.
"ReactOS is part of a decade old effort to replicate a Windows
operating system as an open source or free software based system.
It's not the only effort trying to duplicate Windows type
functionality on an open source system..."