If you are producing a form for a rails model object and it has a “has_many” or “has_and_belongs_to_many” relationship of some other simple objects (which can be identified by name), then this plugin might be for you. It uses a select box to add items to the list, and delete buttons to remove them. It’s all client side, using hidden fields, until you submit the form and it saves the relationships in your model.

For more detail, and a demo, see:

http://blog.craigambrose.com/articles/2006/10/09/associated-list-a-little-has-many-widget-for-rails