While unlikely to put the world's largest software company out of business any time soon, a handful of Linux-based companies are trying to cut into Microsoft's bottom line.
The key? These companies mimic the look of Microsoft's products.
Apple has been very good at early promotion of technologies, but they've yet to actually "create" anything. Apple does the same thing people accuse Microsoft of, it's just Microsoft is more cautious and slower to market.
As has been said to the point of being very very trite, "Imitation is the sincerest form of flattery."
While Apple may have been more popular with their GUI first, I remeber my Tandy 1000a had a GUI of some sort back in 1985. Deskmate? Yes, it was called Deskmate.