Spaz is a free cross-platform Twitter client. It is intended to be a simple, utilitarian desktop app for posting updates to Twitter, as well as viewing updates from friends, followers and the public timeline.

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.

It looks like this one is in early stages of development (they all are really though aren't they!). Anyway, there are five Windows Twitter clients to choose from in the forge now. This one is called triQQr. Strange name. Their website says it is a Twitter submitter tool for windows. They also have a blog here.

Looks like they are from Germany but the site has English too :)