Software from Netscape Communications

The C API Directory SDK, version 1.0 and version 3.0 (beta). This is supported for a number of platforms, and comes in two version, with or without SSL.
A beta version of the v3.0 C SDK, with source, is now available! This is released under the Netscape Public License. You can get all Mozilla source either using FTP, or check out a copy with CVS. As a compliment to the C-SDK, you can now also get the free PerLDAP module!
Netscape also provides a Java LDAP SDK, for free. It now also comes with source, from the Mozilla web site.

Other Free and Open Source software

Luke Howard's LDAP Name Service Switch package, for glibc and Solaris.
University of Michigan's Directory Server, v3.3. This is quite old, and requires patches and some patience to get it to work. I prefer Netscape's Directory Server, it's much better (but obviously not free).
The OpenLDAP project released it's first version of a Umich derived LDAP server recently, simply named openldap-1.0. I haven't tried this software, but it does include a number of fixes, and it should compile and run on most Unix versions.