Twitteron is a nice UI that is skinnable with numerous features inside. The site lists most of the features:
* The ability to run on any OS that supports Adobe AIR
* A smooth, and rich graphical interface
* Browser-like navigation, with back, forward and refresh capability
* One-click access to uploading pictures to TwitPic!
* One-click access to shrinking links
* The ability to search all of twitter with the built in search box!
* The ability to see larger pictures of friends/followers of yours and others!
* Notification capability, with fine tuning of the rate of requests
* Display of your rate limit, along with the reset time
* Adjustable TwitPic/Photo sizes
* Choose an endless amount of colors with custom hue/saturation/lightness sliders
* Custom search notifications!
Twitpics are all rendered inline, no more jumping out to a webpage or relying in popups. That was about the whole list. I did not load the client but a live stream (using tweetgrid) showed numerous people talking about it. Most good, some were not happy. Remember this is still in beta, with no updates since May, which worries me.