The oddly named Spaz is a desktop Twitter client which is available on Windows, Mac and Linux. The utility has a lot of features and is incredibly easy to customize due to it's coding from Javascript, XHTML and CSS.

Spaz Features:
1. No ads
2. Cross-platform: available on all AIR-compatible platforms: Windows, OS X, and Linux
3. Open source software using a New BSD-style license (source available at Spaz Google Code project site)
4. Written in pure Javascript, XHTML and CSS. Utilizes the powerful jQuery Javascript framework
5. Built-in global search
6. Short URL creation tool with support for multiple services (is.gd, bit.ly, snurl.com, and more)
7. In-line short URL decoding
8. Markdown syntax support
9. Multiple themes and support for user-created themes
10. User-defined CSS overrides
11. Event sounds using the Tokyo Train Station soundset by Dominik Dimaano
12. Directory listings of users you're following, and your followers.
13. Debugging and development tools and debug logging

Because Spaz runs using Adobe AIR you bet it's cross-platformed.

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