Ever since Active Directory was introduced with Windows 2000 Server, Microsoft has had the upper hand for enterprise account management. Active Directory provides central administration which is essential in any enterprise or corporate environment. But, the open source community responded with a product that is full featured, and very powerful. It's currently called the "389 Directory Server", and is developed as part of the Fedora Project (Red Hat's free Linux operating system).
Originally, it was called the Fedora Directory Server, introduced in 2005, but it was later renamed to 389 Directory Server. It can interface with Microsoft's Active Directory, and since it is LDAP based it is extremely fast and powerful, and can be interfaced with from other devices that support LDAP authentication. Even client authentication is possible (in Linux, configuring PAM to use it as an authentication service). It also offers a full featured console for easy remote administration.
A full list of features can be found on the Fedora Project page for 389 Directory Server:
And screenshots are also available:
I have been quite impressed at the number of features and functionality of 389 Directory Server. Successful migrations from Windows Active Directory to 389 Directory Server have been documented. Finally, an open source solution to closed source Active Directory, without the hassles of Windows maintenance.