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.
To reply, favorite or retweet a message, hover over it to reveal those options. To write a new tweet, click the "+" in the upper right-hand corner and put the window away by clicking the "x" on the left.
Updates come quickly and the color-coding makes it easy to find what's new. Unfortunately, tweeting is where I ran into trouble. When preparing to write a message, a new window pops up*. In my experience this window took a good 10-15 seconds to load, which interferes with the spontaneity of Twitter.
Also, the badge lists number of unread tweets, but doesn't differentiate between replies and general tweets.
Other than that, this young app (it's currently at version 0.2) is promising and a nice example of a useful Safari extension. The UI looks great and gets out of the way when you want it to. Keep an eye on Ostrich; it's only going to improve.
*I like to force all new windows to open in new tabs, and this was pushing Ostrich's compose box into a new tab as well. To fix this, I went to Safari's Tabs preference and set "Open pages in tabs instead of windows" to Automatically instead of Always.