Twobile: A Twitter client for Windows Mobile
Twobile is a Twitter client for Windows Mobile devices that adds many of the features you'd get from a desktop Twitter client like Twhirl or Twitterific. The application is available as a public beta, and is freeware, although the developer asks anyone who plans to use Twobile for business use to pay a license fee.
The client allows you to update your status, view the public timeline, your friends timeline, send and receive replies and direct messages, view featured users and favorites. And you can view avatars when you click on an individual message.