I just downloaded a little application from the boys and girls at the IconFactory called Twitterrific. It's an OS X app that allows you to post Twitter postings from the desktop without launching the browser. To be honest, I though it was going to be a Dashboard Widget application, but I'm cool with it being a stand-along app.
Twitterrific on the Mac
It's a great little application! Nice, clean, no-nonsence interface with the ability to post 'twits' (if that's the correct word) to your Twitter account as well as view your friends messages. Overall, nice execution, good features, clean design, easy to use - 2 thumbs up!