What Are Some Good Open Source Alternatives to Active Directory?

I'm looking for a good open-source alternative to active directory that can handle:

• Authorization/Authentication
• Group Policy
• Replication and Trust Monitoring

In addition, are there any consolidated systems out there that handle these responsibilities?

Edit: Since a lot have asked for more details, I am trying to offer a service setting up an infrastructure for organizations, hardware/software setups, right now I am looking at a Linux stack, both desktops and servers, however a hybrid stack is possible, and I am investigating alternatives.

FreeIPA is one project that aims to replicate much of the functionality provided by Active Directory.

www.freeipa.org

Samba can do some of the things that AD can, but I'm not sure I'd call it a full-blown alternative. Take a look at this Samba intro to see if it will suit your needs.

I use GoSa as my AD server :

GOsa² provides a powerful GPL'ed framework for managing accounts and systems in LDAP databases. Using GOsa² allows system administrators to easily manage users and groups, fat and thin clients, applications, phones and faxes, mail distribution lists and many other parameters. In conjunction with FAI (Fully Automatic Installation), GOsa² allows the highly automated installation of preconfigured systems. GOsa² therefore provides a single, LDAP-based point of administration for large and small environments, thus making the administration of users and systems and all related parameters manageable and easy.

More info on https://oss.gonicus.de/labs/gosa/

There is no open-source alternative that is even close to the functionality of Active Directory, as of 2009 anyway.

As MrDenny commented on your question - if you need all that, just use Active Directory, assuming you are supporting Windows clients.

I think you could have a look at open-source Apache Directory LDAP v3 compliant server http://directory.apache.org