Lounge

Lounge has a very Mac-like interface, using a single window with a left sidebar. It makes it very clear and easy to get caught up and send new tweets.
You can have multiple accounts, search, and view lists of your friends and followers. But there are deeper functions hidden in the interface as well. For instance, click on the user’s icon and you’ll get their profile and buttons to view their tweets, friends and followers.
Lounge is a native Mac application, not needing Adobe AIR or anything else. It is listed as still in beta and is free to download.
Get Lounge