Twhirl is a twitter client that runs on Adobe AIR. I am using for quite some time and i liked it very much. One of the coolest features of twhirl is that it supports multiple twitter accounts, particularly useful people who maintain different accounts for work and personal.
Twhirl latest version now supports Pownce and Jaiku. When you post to twitter it can automatically post to pownce and jaiku provided you enter the login information.
Using twhirl you can search twitter users, follow them or unfollow them. It will sit in the system tray like a desktop app, you can open it whenever you want in a click. It shows a small notification messages whenever it finds tweets. It will automatically fetches your friends updates, direct messages in to all-in-one timeline. Messages will be color coded to easily recognize replies, updates and direct messages. You can also change color scheme of the twhirl.
It works with both Windows (XP/Vista) and Mac. If you are looking for a good twitter i would recommend using twhirl. Best of all it is free. Via Downloadsquad