Tapbots have released a public alpha of Tweetbot for Mac. If you’ve ever used either of their iOS versions of Tweetbot, you’ll feel right at home with it on your Mac, as Tapbots have maintained consistency.
The public alpha is free to download for now, but it is far from a finished product. Expect bugs, crashes, and incomplete features. I imagine that when it hits 1.0, it will be a paid product in the Mac App Store.