This is probably only of historical interest but: in 2001 I started building a Linux-based Active Directory replacement, XAD. I first demonstrated it in 2003 and I think we shipped around 2005. The underlying technology was OpenLDAP and Heimdal, but obviously with a lot of homegrown code (many RPCs, support for multimaster replication, etc).

XAD was sold to Novell in 2007 and was rebranded Domain Services for Windows. I haven't really followed its progress since; of course, Samba4 is also now an effective Active Directory replacement.