Kiwi is a nice Twitter-App for Mac OS X which will finally replace Tweetie (where i am still waiting on an update or even a beta promised by MacHeist – boo on them) for me. It comes as a free version sponsored by ads displayed on top of your timeline or as a paid version for $9.95.
Kiwi has some very nice features like filtering, tabbed-accounts and themes, yet is still clean and not overloaded.
Being used to Tweetie, i haven't really liked any of the built in Kiwi-Themes too much and immediately whipped one up which you can see to the left and can download here if you like.
After downloading the Theme, simply unzip and double click to install.