Bitter is a free client that integrates Twitter, Plurk, and Friendfeed social networks. It gives you one integrated place to track your friends’ activities in different social networks. Instead of using a separate client for each of these, Bitter lets you access Twitter, Plurk, and Friendfeed from one place.
Bitter is a free desktop client. It is very easy to install, and is small in size. Once installed, you will really like its single column UI. You can manage multiple Twitter accounts, and it shows your number of unread tweets, mentions, direct messages, and favorites.
Apart from that, Bitter also lets you reply, reply to all, and send direct messages. It even comes with a built-in URL shortener, and also lets you post pics on Twitpic.
All in all, Bitter is pretty good, and very useful for those who are active on Twitter, Plurk, and Friendfeed. If you want a client for Twitter and Facebook, you can use Twigether, Brizzly, or SocialVisor.