Buzzbird is totally free (as in beer and freedom). There are no advertisements, and it is completely open-source. A lightweight Twitter client. Buzzbird is an open-source, MIT licensed Twitter client built on the Mozilla XUL framework. It does not use Adobe AIR, and runs on Mac, Linux, and Windows. Most of the code is written in Javascript. All you need to do is install it and login to be able to manage your tweets.

Buzzbird Features:
1. Multiple Account Support
2. Selective filter views (can choose Timeline, @mentions, direct mentions)
3. Retweet, Reply, Unfollow, and Favorite button on each Tweet
4. Built-in URL Shortening (currently via is.gd)
5. Menu to insert emoji in tweets
6. Read/Unread count in status bar
7. Click on a users avatar to see more information about them, or to follow/unfollow them.

Buzzbird is available for Mac OSX, Linux, and Windows. Buzzbird is 100% Free, 100% Open Source
You read that right. Free as in beer. Free as in freedom. Free as in ad-free.

No Third-Party Runtimes
Adobe AIR? Never heard of it. JavaFX? Nope. If you want to run Buzzbird, you install Buzzbird. That's it. No bulky third-party runtimes to update and maintain. Buzzbird 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.

The license of this software is Free, you can free download and free use this social networking software.