I was a big Active Directory fan a ways back. Not for the usual reasons, but for application specific data. After dealing with the fiasco that was the LDAP store in Site Server, it was nice to see a large-scale robust LDAP store. The problem was that the data store was tied to the domain model too tightly.
With that in mind, I was very happy to see that Microsoft noticed and has released Active Directory Application Mode (ADAM). I really like where it is going. While the tools for managing the ADAM stores are pretty deplorable, the data store is pretty solid.
For the LDAP uninitiated, LDAP stores (like ADAM) are made to store non-volatile hierachical data and are tuned for reading, not writing. LDAP is not a replacement for databases, but a different data tool in your toolbox. It has this very specific purpose. What is nice, is that LDAP is a very mature standard. LDAP v3 is a protocol that has been with us for a quite a long time and implemented across the industry (IBM, Sun, Oracle, Novell, etc.)