Twitter's free client for Mac has received its first major update today, giving users the ability to view multiple timelines at once. Other improvements include easier navigation and auto-completion of both hashtags and usernames.
Viewing multiple timelines happens in multiple windows, unlike previously mentioned TweetDeck, which uses columns inside one large window. Navigating back through timelines is now easier due to a bar at the top of each window, and they've also added a "New Tweet" button to the main app window. [more]