ActiveScaffold

ActiveScaffold, Acts_as_taggable_on_steroids

Update: also read Active Scaffold + Acts_as_taggable + Auto Completion. This is kind of an advanced topic, but I think it may be useful to a lot of people. ActiveScaffold is a great plugin to start building a user interface. The great thing about AS is, that is automatically recognizes associated models. When editing a model, you can easily add or select another model that you want to associate with is.