Tweetscroll
Automatically show new tweets on Twitter web interface- Drag Tweetscroll to your bookmarks bar
- Go to Twitter.com and click your Tweetscroll bookmark
- Done. New tweets will start showing up automatically ;)
Tested on Chrome 12, Firefox 5, IE9, Opera 11 and Safari 5.
See also 
User script, 
Chrome extension, 
Opera 12 extension