Open Directory is an open source back end with a closed source schema and front end.
One thing to consider is how much time you spend on the solution and it's usability afterwords. If your solution needs too much support or is onerous to use than you save a little upfront but are spending more in the long term. In many cases the tools are more important than most other considerations.
In general you can use almost any open source LDAP server for a simple network. OpenDS and 389 Directory are easy to setup and have tools to simplify adding users, machines, groups, etc.
If you have complex needs something like FreeIPA may fit your needs a bit better. I don't have much experience with it but it appears to include the parts needed for an integrated solution and the tools required to make it supportable.