The very first software I looked to download that didn't come pre-installed is a Twitter client. Between my home desktop PC and my work laptop PC I've played with Seesmic Desktop and Tweetdeck but for the most part I've avoided the power features and found the interfaces unattractive. For my Macbook I decided to stray from the Adobe Air apps and go with a Mac native client: Tweetie. A week later and i'm pretty happy with the choice.
It may be superficial but the number one reason I like Tweetie is its slick appearance (and really aesthetics is one of the reasons people love Mac, no?). It behaves like and iPhone app, sliding gracefully from window to window. It also has a clean interface for accessing your @replies, DM's, and managing multiple Twitter accounts seamlessly. It's not a power tool - you can't build custom groups, and you can't make columns - but I am much happier on Tweetie so far than I was in the overwhelming information stream of Tweetdeck.
Improvements I desperately want from Tweetie: There is no easy way to retweet people's messages with 1-click, and when you do it from the top navigation menu it defaults to the (Via @....) method rather than the more common "RT @...". We should have the option to choose which we want, and RT in 1-click. Also, I definitely want an easy way to pop to the top of Tweetie to see the most recent messages rather than it fixing to the last place I read - at some point when you return to your computer after a while you give up on catching up and want to go to the most recent. Hopefully Tweetie keeps up with the app and rolls out improvements, while keeping with what makes it great - the clean, pretty, and simple.