Tw-autocomplete is a Firefox plugin adds autocomplete support for @ messages and direct messages to Twitter.com
The plugin is released as open source and the code can be downloaded from here.

BuzzGrowl lets you see what others are saying as you surf the web or shop online. It's a browser add-on...
Twitlet is a JavaScript bookmarklet for updating your Twitter status. With its simple interface (a JavaScript prompt window only) you...