Weet Twitter client for Mac hits beta, already looks promising
The general line on Weet (both the iOS version and the Mac beta) is that it's what Tweetie would have become if it hadn't been bought by Twitter. Certainly, the UI of Weet for Mac is more compact than that of Tweetie for Mac -- Weet ditches the sidebar navigation for buttons on top, for one thing. The minimal blue-and-white theme is easy on the eyes, too. (Serious theme junkies will still prefer Kiwi, though.)
Functionally, Weet has some nice perks. There's a mute option for those loudmouths you can't bring yourself to unfollow, and a text filter to block tweet topics you can't stand to read any more about. Saved searches and lists are supported, too. Weet is still missing Tweetie's very useful "go to user" option (the iPhone version of Weet has this problem, too), and it's got plenty of bugs, even for a beta. Still, the Weet iPhone app has steadily improved and become my go-to Twitter client, and the Mac version is poised to do the same.
One last note: this version of Weet for Mac seems to be Snow Leopard only, so Leopard users are out of luck for the moment.