Twitter for Windows Phone app is now available to all Windows Phone 7 devices, and includes features such as: your timeline, suggested users, messages, lists, and a great way to explore Twitter without even logging in. You can sign up for an account right from the app as well.
The app uses a slick-looking new "Metro" interface, powered by Win7's Pivot feature. Pivots allow you to swipe left or right to view different categories or types of information within a single app.
It also appears to include advanced Twitter features, including viewing photos and locations.
The app was created using an open source REST API called Tweet Sharp, and that the developers at Identity Mine improved upon the code and contributed it back to the open source project.