Probably not an OS, but is a program that you are looking at.
It's Wine. It allows you to port Windows native program over to
Linux, using the Windows API. Its source codes are 100%
non-Microsoft, although there's no programs with getting Microsoft
files (especially DLLs to work).
Wine Home
Page
Some OS with great compatibility with Windows programs (but please
don't directly port it over, it might not work)
OpenSuse
Fedora
Linux XP
Freespire
Linspire (paid version of Freespire)
Slynux
ReactOS
Mandriva (previously Mandrake)
They look and work so much like Windows that you probably never
realize you are using Linux.
A few others that will work (with the help of other open-sourced
programs):
Ubuntu and its derivatives (Kubuntu, Edubuntu, Xubuntu)
Debian
Knoppix
Gentoo
MEPIS
PCLinux
PC-BSD
CentOS
Damn Small Linux
Puppy