Twittastic is a FREE twitter client based on .net framework which has a quite clear, minimalistic and a sleek design as a twitter client. It offers some amazing features like you can directly update your personal profile on twitter, take a screenshot and share it with your twitter followers, upload a picture directly to Twitpic just by drag and drop over the application.
Shorten the URLs you want to share on twitter on the go, you can also take webcam snapshots and upload them to twitpic and share them with your twitter friends.
After Tweetdeck this is the second best client I would love to use, moreover you can do lot more amazing things like sending a file to any one via twitter with this application, the sent file will be shared through drop.io