Ambientweet for Mac

From Elegant Chaos:

Ambientweet is a minimalist twitter client, and it is intended to just be left running in the background, as a way of watching your twitter stream pass by. Ambientweet shows just one tweet at a time. This is a deliberate design decision, which allows for a minimal user interface. This, in turn, means that the text can be bigger and there's no need for extraneous clutter like toolbars or buttons whilst you watch the twitter world go by. You can use Ambientweet to post, reply and re-tweet, so it's perfectly adequate for adding to your stream and responding to what you read. What it doesn't include is lots of complicated functionality for viewing profiles, following people, managing lists and accounts, and so on.