Is LDAP the answer?

[Kimball Larsen]

Perhaps I'm missing something massive out there (LDAP, perhaps?) but  
here goes:

Our organization currently uses Exchange Server for the sole purpose  
of sharing contacts in Outlook/Entourage on the desktop.

Is there a free/os linux tool that can do the same thing Exchange  
Server is doing for us now?  Ie, just supply a way to let Outlook/ 
Entourage see a shared list of contacts that everyone can update?

Thanks!

-- Kimball