Comment freeIPA (Score 2, Informative) 409
If you are looking merely to replace or emulate the ldap/kerberos functionality of AD you could take a look at freeIPA , a project under active development, sponsored by Redhat and based on Redhat/Fedora Directory Server, but with an enhanced web-GUI and some additional functionality
From my experience, in a small-to-medium Linux/*BSD/OS X environment, with NFSv4 or AFS, this will work fine.
However, as other posters here suggest: if you have predominantly windows clients, for your own sanity it would be better just to use AD from the outset.