Twhirl is a very good example. In this case, the interface is not the only remarkable thing in Twhirl.
Twhirl is an application that allows you to manage your Twitter account easily and know the status of your contacts, view your private messages, manage followers, mark twits as favorites or eliminate them...
Finally, you can use more than one account and set different sounds for each one, separate twits in tabs, etc.
It requires Adobe Air.