Although not used myself, Suns OpenDS: http://www.opends.org/

Just for reference, other directories:

  IBM Tivoli
  Microsoft Active Directory
  Novell eDirectory
  Red Hat Directory Server
  Critical Path Directory Server

Fedora DS is an open source fork of Netscape 5.1. RHDS is basically the same product. High-performance -- if you need tens of thousands of accounts, hundreds or thousands of simultaneous queries, a netscape derivative is what you want.

Sun ONE DS is free-as-in-beer. It's a newer derivative of the same codebase (Netscape) that became FDS.

Apache DS is something I've been experimenting with. Java-based. Easy to get up and running, comes with a really great default schema.

Sun OpenDS looked cool, but since the oracle acquisition, I can't bring myself to be interested in it.

Also: http://directory.apache.org/