Twidget is a free Dashboard Widget for OS X that allows you to update your Twitter status.
Displays 20 most recent public or friends timeline
Auto-refreshes timeline display
Easily show/hide timeline display
One-click refresh of timeline
Links to reply and Direct Message Twitter contacts
Link to mark tweets as "Favourite"
Recognizes replies and auto-links them to follow conversations
Shows your last tweet regardless of how it was updated (ie: mobile, txt, etc.)
Auto-notification of new versions of Twidget