While an individual user may use Twitter primarily as a conversational tool or a broadcast medium, in its totality, Twitter operates a lot like a wiki: as a knowledge-sharing, co-creation platform that produces content and allows its consumption. Conversation is perhaps the most simple and obvious form of collaboration, but would anyone claim that Wikipedia is a conversational platform? Despite the presence of information sharing, co-creation of an end product, and even discussion pages, Wikipedians on the whole aren't having conversations. According to this argument, Twitter is no more a conversational platform than Wikipedia is.
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.