I just downloaded and installed Seesmic Desktop Application and I must say it is highly recommended. You can manage multiple twitter accounts, and send messages from multiple twitter accounts without having to logout and login. Here is a video I made to show how you can add multiple accounts to the application and effortlessly toggle back and forth between the two accounts.
It does alot more, but I figured I would just give you a small sample of what is capable with this powerful application.
Here is a video Seesmic put on their site explaining alittle bit more about the Seesmic Desktop application. Visit http://desktop.seesmic.com/ for more info.