Mac OS X: If you're looking for a simple Twitter desktop application without the bulk, Itsy is a great new free option for your Mac.
Itsy is super simple. You just log in and up comes your Twitter feed. You can switch between the main feed, your @ replies and direct messages. Itsy also supports search and of course posting. It sits in a tiny little window on your screen and keeps out of your way. If you want to show and hide it, it has a little menubar icon to do just that. Itsy is completely free and a nice way to manage your daily Twitter needs without too much distraction.