As noted by John Gruber at Daring Fireball, the simplicity of a Twitter post opens up a lot of UI opportunities. As a result of that and available APIs, we are seeing more and more Twitter clients (I have four on my iPhone), but beyond that, we are seeing the start of the next logical step, convergence.
Socialite and Raindrop are the two best examples that I know. These are applications that aggregate your social footprint on the internet and attempt to wrap some meaningful UI around it all. Raindrop is still in early dev and requires you to be a bit savvy if you want to use it, but Socialite is (Mac only and) immediately available and is quick to set up, so give it a try.
Something worth looking at when using a variety of Twitter apps and these convergence apps, is how the designer has chosen to organize and present everything. If you boil things down a bit, a tweet, blog entry, Facebook update, Flickr post, rss feed and email, all have the same basic parts...
With such a simple set of elements, there is a ton of opportunity to design a look and feel around them. The Raindrop folks have been sharing their design ideas on Flickr and you can see they have been exploring many ideas. It's not a surprise to see a 3 pane UI like you have in Outlook, Mail.app and NetNewsWire.
Socialite has completely Mac-ified that experience, and while some folks think the UI is just too intense, I think it's done elegantly and sensibly. You can see though that there is a lot you can do in the app, especially on the right side of each entry. At times there are 5 icons and one of them has a menu with 15 actions you can take. But overall, the UI is based on a use case you already understand, email.
Back to Twitter clients, compare something like Tweetdeck (which is entirely consistent from iPhone to its Air counterpart) to something like Tweetie 2. The base philosophy of these two clients are very different, but they use the same source material. As Raindrop continues to develop, it will be interesting to see if they can find a different but equally (or more!) usable format than you find in Socialite.
For a good overview on Socialite, check out Mashable's article from last week.