Lori: As Kevin mentions, there are several choices available for managing and authenticating users and directories. If you are an existing Novell shop (or even if you are not) you may want to implement NDS eDirectory, which is available for multiple platforms and natively supports LDAP. This is an inexpensive solution -- just $2 per user; Novell also has an NDS Corporate Edition at $26 per user that offers more features for managing network resources.
For those wanting to go the Microsoft route, Active Directory is built into Windows 2000 and lets you manage users, servers, and directories. Active Directory supports LDAP natively over SSL (secure socket layer). Active Directory makes administrators lives easier with its centralized management of resources and users.
NDS is also available for Windows NT/2000 servers, so if you are heavily invested in NetWare you'll find this attractive. A good deal of information can be found on their Web sites; you can compare the benefits of both. In either case, much planning is required for implementing an LDAP service and especially for Active Directory, which is not for the faint of heart.
If you are not ready to invest in the stand-alone model, then I suggest using Active Directory or NDS.