Tweetings is a small, secure Twitter client for Windows. It has a compact interface that scoots out of the way when you minimize to the system tray.
TwInbox was originally named OutTwit. It is an Outlook plugin that integrates Twitter into the Outlook interface. From the TwInbox website:
From digsby.com:
digsby is a multiprotocol IM client that lets you chat with all your friends on AIM, MSN, Yahoo, ICQ, Google Talk, and Jabber with one simple to manage buddy list. digsby also has a social networking tool that alerts you of events like new messages and gives you a live Newsfeed of what your friends are up to.
Bitter is a lightweight Twitter client built on .NET, so it's Windows only. From the Bitterware site:
It consumes a low amount of memory, is theme-able, is secure (uses HTTPS for authentication and encrypts authentication details), uses a familiar tabbed-interface and so much more!