Ambientweet is a minimalist twitter client. The design puts the emphasis firmly on reading tweets, not posting them, and it is intended to just be left running in the background, as a way of watching your twitter stream pass by. Ambientweet intentionally shows just one tweet at a time. This allows for a very minimal user interface in the main window, which means that the text can be bigger and there's no need for extraneous clutter like toolbars or buttons. Eventually I may implement some more complex twitter features such as direct messages, and allowing you to post fro Ambientweet, but at this stage the plan is to keep the focus squarely on having something clean that you can use to watch the world go by... One thing that I probably will also implement is a theme system of some sort, so that you can customised the appearance of the tweets.
See http://www.elegantchaos.com/ambientweet for a full description of Ambientweet.