What it is: A multi-account Twitter client, available for iPhone and Mac OS X.
Why it’s good: Both versions of Tweetie succeed in marrying a usable UI with a strong feature set. Although Tweetie for iPhone and Tweetie for Mac share some aspects of design, both play to the strengths of the host platform. On iPhone, Tweetie makes the most of the touch display, and its efficient UI means there’s never any stuttering. On Mac, Tweetie has keyboard shortcuts for practically every action, and its sidebar deals with the thorny issue of multi-account UI without resorting to tabs. In both cases, the app is feature-rich, providing a great experience for most Twitter users. The 1.1 update also brings saved searches, Growl support, and a bunch of other tweaks and fixes.
Where to get it: Tweetie for iPhone is available on the App Store for $2.99. Tweetie for Mac is available from atebits.com. By default, Tweetie for Mac is supported by unobtrusive and surprisingly relevant ads, but you can make them optional by paying $19.95.