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 in the stream without having to click through links.
While Twitter has been less than forthcoming on how they plan to monetize their service, there is no shortage of ideas from third parties on ways to get paid for spending time with Twitter.