File under open source LDAP.

Brian K. Jones has blogged about his experience with the Fedora directory server. He likes it.

In contrast, the Fedora directory server has huge, enormous, steaming wads of documentation, a wiki that has a huge amount of more task-specific documentation written by those in the community who waded through one project or another and lived to tell about it, a mailing list that is extremely user-friendly, and even an IRC channel where you can talk directly to some of the folks writing the code, who are an immense help, and whose wisdom often makes it into the published wiki documentation for all to benefit from.

So, in short, Fedora directory server is a blazing fast directory server that supports multimaster replication (should you choose to use it), hot backups and restores, access control changes (and many other changes) without a server restart, running multiple instances on a single machine, and it stores its entire configuration in the directory itself, making it completely manageable using the LDAP protocol itself. If you’re a GUI fan, there’s also a graphical interface that lets you do everything from adding new users, to adding new objectclass and attribute definitions, to managing certificates and viewing logs. What’s more, with its PassSync utility, it can synchronize passwords easily with an Active directory server.

That about sums things up I’d say. Though, if you would like full enterprise support I recommend the Red Hat badged version.