Witty is a free Twitter desktop client for Windows Vista and XP powered by the Windows Presentation Foundation (WPF).

Witty includes the following functionalities:

+ View recent Twitter statuses from the people you follow (auto-refreshing)
+ View recent replies to you
+ View a specific user’s timeline
+ Update your twitter status
+ View and respond to Direct Messages
+ Links will open in the user’s default browser

Other optional features include the ability to change the auto refresh interval, skin, sounds, and a number of keyboard shortcuts for quick navigation.

Download the beta version of Witty for Windows Vista and XP here.