*Chirp is a WPF desktop widget for reading twitter. It's very slick. Not as feature filled as Twittastic or Twhirl but the UI is much faster and more aesthetically pleasing.
I do wish they'd make the logo in the upper left corner a bit smaller.
Be sure to install it for .NET 3.5 SP1, it runs a lot faster than the 3.0 version:
Get it here.