Tweetie for Mac has some great keyboard shortcuts

Tweetie for Mac lets you switch between the Timeline, Replies, Direct Messages, and Search panels of your account with Command-1, -2, -3, and -4, relatively speaking.

If you use more than one Twitter account in Tweetie, you can switch between them by using Command-Shift-Up/Down arrows. This means you can switch between any account, and any panel under each account, without touching your mouse.

Tweetie’s shortcut convenience extends even to its popup posting window: hit Command-Down arrow while drafting a tweet, and you can change the account that you’re posting to by arrowing up and down, then hitting Space or Return.