Tweetree puts your Twitter stream in a tree so you can see the posts people are replying to in context. It also pulls in lots of external content like twitpic photos, youtube videos and more, so that you can see them right in your stream without having to click through every link your friends post
So I logged in and played around some. It worked right away showing threads, pictures and even highlighted links.
Draconis Software out of MA is the designer behind the site and it seems they have done some excellent work. One to watch.