Well looky here.
Twitter, Inc. just released version 2.1 of its Mac OS client via the Mac App Store. Twitter 2.1 is the first major update of the software since Twitter acquired Tweetie (and its publisher Apebits) in April 2010.
The biggest new feature in Twitter 2.1 is multi-window support which allows you to view multiple timelines at once by selecting the new "Open in new window" menu item (cmd-shift-T).
The single window interface was one of Twitter for Mac's biggest limitations and 2.1 instantly makes the app more appealing to users with multiple accounts and those tracking multiple timelines. Multiple windows puts Twitter for Mac more on par with a desktop client like TweetDeck.
Other notable features include:
Twitter for Mac is available for free from the Mac App Store.
Tip: Lifehacker