There are a number of Mac friendly Twitter clients available, but where many fall down is in the looks department. Nambu gets off to a great start by closely tying in with the look of OS X and offering a streamlined, graceful interface.
A great deal of thought has been put into the usability of the program and as such you'll find features such as the displaying of the full address of shortened URLs and thumbnails of pictures that have been posted by others.
Little touches such as the autocompletion of names when sending messages to others and a conversation view to make it easier to follow a series of messages, all help to make Nambu a pleasure to use. With a built in translation tool, foreign tweets can be understood in a flash
With support for the new and old methods of retweeting, inline display of messages retweeted by those you follow and the ability to work with multiple accounts - more than one account ca even be displayed as a single stream -, Nambu is everything you would expect of a Twitter client and more.
Nambu has all the Twitter basics covered, but benefits from a wonderful interface and plenty of extras.