One way Twitter has started to be used is a way of picking up what's going on in the world, like a micro news service.
Yahoo! have created Sideline as an application to keep abreast of the Twitter-sphere. Unlike most Twitter clients, Sideline is only a reader and you can't tweet from the application, although clicking any tweet will open your browser.
The first tab on Sideline shows topics currently "trending" on Twitter, and you can click on any trend to see the most recent tweets. Refreshing will update the list, while clicking the add button will ope that subject as a new tab, so you can read more tweets on that subject. If you like a certain Tweet, click the favorites star, and that person will be added to your favorites list.
Many Adobe Air based products are very attractive, but Sideline is pretty average. In terms of functionality, it feels unfinished, as just reading Twitter with no ability to tweet yourself seems limiting. The trends feature is interesting, but you can't add your own filters yet. You can also create your own topic searches.
Sideline needs a lot of work to become a useful Twitter client as there are currently websites that do Twitter trends better and are better presented.