Today we got our hands on a beta of Lounge, a new client for Mac OS X (and iPhone/iPod touch). It's definitely got a way to go (I saw more beach balls than Memorial Day at Hilton Head), but I see where they're going and I like it.
First off, there's support for multiple accounts, and switching between them is easy. A series of buttons appear horizontally in the left of the window, including "My 2 Cents" (which displays just your tweets), Favorites, Friends, Replies and my favorite, Vanity, which finds all occurrences of your Twitter handle.
Mouse over a user's tweet to see options to re-tweet, copy the URL, open in a browser and more. I really like the option to view user details, which lets you see just that user's tweets, his followers, those being followed and profile and URL.
It is kind of wide (wider than Twitterrific but not TweetDeck, of course) so those with small displays be warned. Also, some may be put off by the pastel-colored icons. Still, it shows promise. Check it out if you're up for it, and provide some feedback to make Lounge better.