rolify 3.2 is out - Role management library with resource scoping w/ Mongoid support
rolify gem allows you to manage the roles in your app easily and provides the ability to scope the role on a resource instance or class:
user.add_role :moderator, Forum.find(3)
It integrates smoothly with Devise and CanCan to build a full authentication/authorization stack
3.2 release includes new handy features like callbacks when roles are added/removed, helper methods on the resource class and the User class to find resources/users depending the roles they have, and lot more !