Turba is the Horde contact management application. It makes heavy use
of the Horde framework to provide integration with other applications.

Turba is a fairly complete basic contact management application.
SQL and LDAP backends are available.
You can define the fields in your addressbooks in a very flexible way,
just by changing the config files.
And there are Horde API functions to add and search for contacts.

WWW: http://horde.org/turba/