LDAPman Directory Server List

There are more LDAP servers out there than you'd think. If I've left off your favorite LDAP server, please let me know.

OpenLDAP -- Are you a do-it-yourselfer? This one is free! Since it's source code, you can compile this for most computer platforms
Messaging Direct's M-Vault LDAP server -- For Solaris, NT, IRIX, AIX, and Linux
iPlanet / Netscape Directory Server -- For Solaris, NT, HP-UX, AIX, IRIX, and Linux
Novell NDS eDirectory
IBM Directory
Microsoft Active Directory -- Warning: Though Microsoft touts Active Directory as an LDAP server, its LDAP support is not fully debugged yet -- their ADSI interface is reportedly stable. If you're expecting to use this in a cross platform environment, look elsewhere.