I'm not a huge Microsoft fan, but Active Directory is one innovation of theirs that's tough to argue with.
All things considered, Active Directory is a very well-thought-through directory system that doesn't seem to be a mere refinement of a competitor's system. At least not when you consider its most innovative features like multimastering. Linux and Unix in general are still playing catch-up with AD and it's been out for years.
Yes, I know about NIS/YP, but it's more appropriate to compare simplistic flat systems like that to old-style NT domains. AD is several quantum jumps beyond that. Who had a really usable enterprise-class distributed hierarchical directory service before Microsoft?
AD does so much so well that it's possible to, for instance, set up intranet secure web servers and have them get their keys automagically through AD. Compare that to the hoops you jump through to do anything similar on Linux.