Buzzbird

Buzzbird is totally free (as in beer and freedom). There are no advertisements, and it is completely open-source. It is built on Mozilla’s XUL platform, the same technology that is used in the Firefox Web Browser, Thunderbird E-mail client, and Songbird Media Player, which means Buzzbird can be multi-platform and still run natively.

Features

Here’s what Buzzbird supports today:
Multiple Account Support
Desktop notifications
Service filtering
Conversation views
Selective filter views (can choose Timeline, @mentions, direct mentions)
Retweet, Reply, Unfollow, and Favorite button on each Tweet
Built-in URL Shortening (currently via is.gd)
Menu to insert emoji in tweets
Read/Unread count in status bar
Click on a users avatar to see more information about them, or to follow/unfollow them.