I think there is no full replacement to MS Active Directory service.
Partly it could be eDirectory servers offered by various companys (RedHat, NetIQ), but these servers are oriented to Linux and related software, and often used only for authentication or storing address book.
I think you should ask the developers of various eDirectory servers about its features and can it software kill the AD in your company.
Oh, look I think this thing represent a big problem for company that wanna move the entire IT deparment to the free software. I am going to still looking for an replacement,however thanks for your help