Ostrich is an extension for Safari 5 that monitors twitter while staying out of the way. Installation is easy. First, make sure you've got extensions enabled. From the Develop menu, select Enable Extensions. Next, open Safari's preferences, click Extensions and turn it on. Once that's done, download the extension, double-click it and give Twitter permission to connect when prompted to do so.
You'll notice that a tiny ostrich head has been added to your toolbar. As new tweets arrive, a numbered badge appears just above the poor fellow's eye. Click it to reveal the Ostrich UI. This simple drop-down displays up to 20 tweets at a time. New arrivals are tinted yellow. Across the top you see icons for your friends' stream, replies to you and your favorites.