Alternatives to Active Directory

Is there any good alternatives for active directory?

To clarify, after deploying and maintaining many Windows-only sites all using Active Directory, I started to wonder why I never thought twice about using something else, e.g. Linux. I understand that a full-featured alternative may not exist yet, but I'm at least looking for something that is aspiring to be active directory and working towards it.

Likewise-Open makes integrating Linux clients and member servers into an existing Active Directory fairly simple. We use it on a couple of Ubuntu servers for NAS -- just upgraded to Jaunty and it works great, though we stuck with the likewise-open (4.x) rather than likewise-open5 package, as there are some changes in the newest version which we haven't figured out completely. In particular, Likewise takes some of the overhead and configuration out of the krb5/pam/winbind/samba setup. Supposedly its authentication mechanism is more efficient too, but this isn't something that we've really noticed.

Also, the long-awaited Samba 4 is supposed to be coming in the not-too-distant future, and promises a number of interoperability improvements such as Group Policy support, might be worth staying tuned.

I'm surprised no one mentioned Novell's eDirectory. It has been around since 1993 (Of course if was called NDS back then). http://www.novell.com/products/edirectory/whychoose.html