The Iconfactory has announced Twitterrific 2.1 beta, a meaty upgrade to their sexy Twitter client that brings quite a few new features and fixes. At the top of the list is support for Growl (don't forget to adjust your Growl settings in System Preferences, not Twitterrific), as well support for multiple Twitter accounts (bust the top of the window in my screenshot), which means Twitterrific no longer shares Safari's keychain item for Twitter; it's using its own Keychain entries now. Also in the What's New text file included with the download (which I highly recommend perusing if you snag a copy) is AppleScript support, NetNewsWire integration (for tweeting headlines you're reading), drag and drop support for creating links to tweets in other applications, a slew of manually edit-able power user preferences (explained in the Read Me) and much more.
This is a great update, especially for the power Twitter users in the crowd. I've been running and tinkering with it while drafting this post and haven't run into any problems, and logging into multiple accounts works like a charm. The original Twitterrific 2.0 version is still of course available for download from The Iconfactory, but the 2.1 beta, as far as I know, can only be grabbed from the Twitterrific account's announcement here.