Lockergnome highlights three major brands that can't seem to abandon the Windows fixation: World of Warcraft, Gametap, and Netflix. The first two I can (moderately) understand. But why should a user be forced to use Microsoft's ActiveX to get the most out a DVD rental service? Does it provide some secret value that having a wider swath of users wouldn't more than make up for?
For those web developers who continue to fixate on IE, you could at least support Firefox and IE equally, and not on any particular operating system. The world is opening up. You should, too. Good design doesn't depend on a particular operating system. That's Microsoft's lock-in schtick. It need not be yours.
P.S. Yes, I know the services above also support the Mac - at least, in part. That's not the point. The point is developing in such a way that other platforms like Linux don't become second-class citizens.