Echofon
Twitter client features a clean interface, the ability to sync with your iPhone, drag-and-drop attachments, a browser drawer and multiple accounts. The free version has ads.
Nambu
Organize your Twitter accounts with saved searches, topic threading, filters and groups. Check out trends in real time.
Seesmic Desktop
Gather your social networks in one window side by side. Included are Twitter, Facebook, LinkedIn, Google Buzz, Foursquare, and Ping.fm — but plugins for others are available. Based on Silverlight, which might be a bit choppy on your Mac.
TweetDeck
Spread tweets into columns spanning the width of your computer screen with sections for friends, mentions, direct messages, trending topics and recommendations. In addition to Twitter, you can sign up for Facebook, Buzz, LinkedIn, Foursquare and MySpace.
Download site
Comment
— 2 so far
Share
Tweetie
Keep track of Twitter from your desktop. Menubar icon turns blue to let you know when new tweets arrive. Interface allows you to switch between accounts and send tweets of your own. Update: Version 2 of this app is simply called Twitter and is available only at the Mac App Store.
Twitterific
Post and read tweets in this app which features and a clean interface, support for multiple accounts and auto-refreshing. The free version has advertising.
Yoono
Connect to your social networks and instant messaging services all in one place. Get updates and share with friends using Twitter, Facebook, MySpace, Flickr, AIM, Google Talk and more. Update your status across all your social networks at the same time. All your IM services in one application.