LDAP

Lightweight Directory Access Protocol. This of it as an Open Source version of Microsoft's Active Directory (although LDAP is not based on AD, and in fact AD can be thought of as LDAP+Microsoft cruft just as well.) Tinkering with it might not make it seem "lightweight" until you consider that its "heavyweight" sibling is the X.500 protocol, about which the less said, the better.

See the wikipedia entry or this excellent introduction or the openldap site