Frostale allows active directory login through a ruby gem.

* A small utiltity library to help alleviate the tedium of managing the contacts and e-mail lists from a Small Business Server 2003 Active Directory.

Authenticate against Active Directory with minimal configuration

Active Directory For Ruby

Ruby bindings for Microsofts Active Directory (LDAP), a fork of ActiveDirectory

ActiveDirectory uses Net::LDAP to provide a means of accessing and modifying an Active Directory data store.

PassiveLDAP is an LDAP-ActiveRecord interoperability library designed to allow an ActiveRecord model (usually the Users model) to be replaced with an LDAP-based model. Active Directory examples included!