Use twitcher if you want uncomplicated, fast and easy to use desktop access to your twitter account. It has many of the normal features you would expect plus some more like ToolTips that show expanded shortened URLs.

Twitterrific is a fun application that lets you read and publish posts or "tweets" to the Twitter community website. The application's user interface is clean, concise and designed to take up a minimum of real estate on your Mac's desktop.

Seesmic Desktop is a cross-platform desktop Twitter client developed in Adobe AIR.

DesktopTweet is a small program for Windows that lets you quickly take snapshots of your computer's screen, and then tweet them up to Twitter in a matter of seconds!

All you need to do is highlight the region of the screen you want to snap, write a quick tweet for it, and then DesktopTweet will handle the rest, uploading it to an online Twitter picture service of your choice! Whether it's a picture of something you're currently working on, or maybe something amusing you found online, DesktopTweet makes it much easier to add extra visual spice to your tweets, quickly and cleanly!