tweet.IM is a free service offered by ProcessOne. It enables you to use micro-blogging platforms like Twitter from your existing instant messaging application. The service makes it easier for you to send messages (called tweets), as well as follow the tweets of other users.
TwitterMoko is a simple client for Twitter http://www.twitter.com. Now support http://identi.ca too
TwitterFox, modified to work with Identi.ca, including cosmetic and subtle code changes.