Tweetstacker.com is a Twitter client that shows users and messages in a threaded display instead of every tweet from every person. It includes replied-to messages in the threads and fetches twitpic thumbnails as well. It automatically gets the latest tweets and adds them to the top of the stream, so there is no need to refresh or click. You can leave the page up all day and always see the latest tweets. This service should be considered beta and still in development. Hit me up on twitter if you have any questions. I'm @justincarlson. Here's a blog post.