Syrinx is my favourite Twitter app. Well designed, not overloaded with options and OSX native. Integrates well with Twitter's webapp if you need more. The "pause" button is a nice option as well.

Two things I would like to see added is tinyurl support and the possibility to have a separate window for replies and direct messages. But I'm not complaining.

PS. The name is nice - maybe not for people from the medical sector, but life is larger than that. They should check the classics. Read up on Hermes, for example.