As an alternative you can use LDAP (Lightweight Directory Access protocol) to provide the authentication data for a number of systems. The OpenLDAP project allows you to download the server and provides documentation.
In the commercial realm there is Novell's eDirectory. Novell has a white paper comparing Novell eDirectory to Microsoft Active Directory that might be of some use. The thing about Novell is that now they are becoming a Linux company and their solutions are likely to embrace open standards. With that said, any product that embraces an open standard limits barriers for other companies to provide more competition. Competition in the space will help prevent your current situation of feeling locked in by a vendor. If you become unhappy with one product you can switch more easily providing the new vendor doesn't work in a proprietary way.
This was first published in January 2005