LDAP has been around for decades and openldap is the only opensource ldap in town until few years ago.
I think openldap is great but it is just too complicated & operationally inconvenience to implement. Along, I have discovered 2 alternative opensource ldap that possible to fit our project requirements.
Alternative 1:-
389 Directory Server aka RedHat Directory Server is available for linux distro but it also work on HPUX, Solaris, etc. It share the common root with openldap with base branch from University of Michigan. It has all the features that openldap offered & it has we admin interface that make the administration easy
Alternative 2:-
OpenDJ/OpenDS is Java based ldap that started by SUN. It probably is the most effortless to setup. On performance, it has reported it can handle millions of search requests per second.
Verdict:-
My choice of opensource ldap is 389 Directory Server as our server farm is mostly RedHat Linux based & our administrators are very much comfortable with it
