Scott Morris has compiled a list of 100 Linux migrations. Needless to say, this list, while welcome, is a drop in the proverbial bucket. There have been tens of thousands of Linux migrations. Linux is so pervasive in enterprises that it's furniture at this point: Everyone needs it. Everyone has it.
Indeed, a more interesting list might be of those companies who have not migrated to Linux, or who tried it and then went back to Windows/UNIX.
Or maybe such a short list is not worth compiling?