Echofon Firefox add-on is a Twitter client that doesn't constantly interupt your browsing
After setting up your account it drops an icon in the status bar of the browser on the bottom right hand side of the screen. Clicking on the icon pops up the main interface, which is simlar to most standard twitter clients with your friend's tweets, mentions, and direct messages accessible each in it's own tab.
So far so standard, but there are a few great features that set it apart especially those taking advantage of it's browser integration. When open, the interface sits over the right side of the browser window but doesn't interrupt surfing.
The browser window keeps focus so scrolling still works, awesome. When new tweets come in a number shows up to the right of the icon showing the unread tweets in the queue, sweet. There's a little button you can click when writing a tweet that will pop in the URL of the site your currently viewing, neat. At regular intervals unread tweets will very unintrusively cycle through at the bottom right of the browser just at the edge of your peripheral vision -- fantastic!
Unintrusive is the key word with this app, and it's made especially so by it being a browser add-on. The need to cycle out of my browser to another application to read tweets isn't conducive to my workflow and growl messages are far too intrusive for things as frequent as new tweets.
Echofon isn't perfect - links aren't automatically compressed, you can't DM someone directly from the feed window, changing accounts appears to be broken and it couldn't be called robust. Those faults aside, it still does a great job of serving up twitter communication without interrupting browsing.