Mac: The alpha version of Tweetbot has received a big update today that adds Tweetdeck-style column view, a new menu bar icon, and a few more smaller tweaks.
The biggest update is the column view. Click the gear icon in the bottom right and create a new window. If you want to snap that window together with the previous one, drag it to the edge of your current column and they'll stick together. You can do this for mentions, direct messages, and other accounts.
The update also adds a new menubar icon where you can quickly tweet or head directly to your mentions. A few smaller updates come as well, including custom URL shortening, a choice of upload services, and improved Mountain Lion support. Tweetbot's still in alpha, and it's still a free download from the developer's site below.