This is YoruFukurou, a Twitter client from Japan, built by @seabream and @aki_null.
It’s a native app for 10.5.8 or 10.6, and packs a lot of features into a nicely self-contained and fairly minimal little package.
One of those features is tabs: you can add Twitter-wide searches or personal filters to the line of filters that appears just below the text entry box.
YoruFukurou (it means “Night Owl”) supports link shortening, TwitPic pictures, current iTunes song, and loads more. It counts your tweet’s characters, it has a variety of appearance modes and adjustable colors, and a decent set of keyboard shortcuts. In short: it’s neat. It really does cater to pretty much all your Twittering needs – well, certainly all of mine, but then I’m not a terribly demanding Twitter user. (Sorry, can’t bring myself to say “Tweeter”. Euw.)
Best of all, it’s free. Go get it.