Scalaris is a Twitter client built as minimal and unintrusive as possible. It shows your friend's tweets as Growl notifications. It runs on Mac OS X 10.4, but later betas (from 1.0b5) require OS X 10.5.

It allows you to send tweets without stopping your workflow. And because Scalaris uses Growl, the customization options are endless. Go into System Preferences and set your custom display styles under the Growl preference pane.