OpenLDAP is described as 'Software is an open source implementation of the Lightweight Directory Access Protocol'. There are more than 10 alternatives to OpenLDAP for a variety of platforms, including Linux, Windows, Mac, Self-Hosted solutions and CentOS. The best alternative is Zentyal. It's not free, so if you're looking for a free alternative, you could try Univention Corporate Server or FreeIPA. Other great apps like OpenLDAP are Microsoft Active Directory (Paid), SambaBox (Paid), 389 Directory Server (Free, Open Source) and GLAuth (Free, Open Source).
Zentyal is a native Microsoft Active Directory implementation on Linux incorporating all the network services required in a small and medium business environment: Directory & Domain Server, Mail Server, Gateway and Infrastructure Server.
FreeIPA is an integrated security information management solution combining Linux (Fedora), 389 Directory Server, MIT Kerberos, NTP, DNS, Dogtag (Certificate System). It consists of a web interface and command-line administration tools.
SambaBox is an integrated enterprise domain appliance which works flawlessly with Windows AD environment. SambaBox offers, Integrated DNS, DHCP, CA and SaltStack with web based management on hight secure environment.
Enterprise-class open source Linux LDAP server. Hardened by real-world use, full-featured, supports multi-master replication. 389 DS can be downloaded for free and set up in less than an hour using the graphical console.
Go-lang LDAP Authentication (GLAuth) is a secure, easy-to-use, LDAP server w/ configurable backends. - Centrally manage accounts across your infrastructure - Centrally manage SSH keys, Linux accounts, and passwords for cloud servers.
Cloud-based directory services. Alternative to Microsoft Active Directory. Provides LDAP-as-a-Service, RADIUS-as-a-Service. Authenticates, authorizes, and manages employees, their devices and IT applications.
ApacheDS™ is an extensible and embeddable directory server entirely written in Java, which has been certified LDAPv3 compatible by the Open Group. Besides LDAP it supports Kerberos 5 and the Change Password Protocol.
OpenDJ is a directory server which implements a wide range of Lightweight Directory Access Protocol and related standards, including full compliance with LDAPv3 but also support for Directory Service Markup Language (DSMLv2).