Pwytter is a standalone Twitter client.

It’s a free/open source software licenced under the GNU GPL.

Pwytter is written in Python/tkInter and is intended to be multi-platform.

TeleTwitter 

We came across this open source project today to make a 'Rich' twitter client for Windows. By the looks of the project page , it is being developed in C and .NET under a BSD license.

It looks like it is quite active and there are about 20 developers attached to the project.  You can download the client from their project page.   If anyone has any comments on what this client is like, let us know in the comments or under TeleTwitter in the forge.  

Open source project to create a Rich twitter client. BSD licensed. From the tags on the project page it looks like it written in .net and C.

An open source project to create a Mac OS X Twitter Client like Twitteriffic