TwitBird Pro is a full-featured iPhone Twitter app that supports landscape view for tweets and the inline web browser. As they claim in the App Store description, landscape view is available on pretty much every screen of this iPhone app. If you are a fan of typing or web browsing with a wide screen on the iPhone, this is the twitter app for you!
Syrinx is a free, customizable twitter app for the Mac. It's interface is simple yet configurable. You can view a column at a time, either your friends, your @ mentions, or your DMs. Some key features: