Twit Menulet is the only Twitter client that fits in your menu bar-allowing you to monitor the tweet stream throughout the day, without opening your browser or another stand-alone application. Tweets are displayed as a scrolling "ticker" in your menu bar. Click the menu to see the whole tweet stream, to visit links, or to post your own tweets.
Twit Menulet supports Twitter Lists and native re-tweets, organizes tweets into threaded conversations, allows multiple accounts, and responds to global hotkeys that allow quick access to Twitter at any time. The application comes with an extensive preferences panel, including the option to set the width of the menu item.
Now, Twit Menulet is also a Tumblr client--and shows photo posts inline!