My Xbox Live! [share your gamertag]
Microsoft just released an official XBOX live app - free. very cool.
it doesnt have all the features that a WP7 phone does... like you cant control the xbox or download games. But its still very good.
Take your Xbox LIVE experience wherever you go with the My Xbox LIVE app. Track and compare your achievements, connect with your Xbox LIVE friends, and change up your 3D Avatar. Review all your recent great games you and your friends love to play and compare achievements with them. Jump into our games hub to learn about the latest LIVE games and apps. Access Xbox Spotlight feeds, get breaking news from Xbox LIVE, game tips and tricks, gamer spotlight and much more.
I have used a few others and found them lacking in one way or another. I had an issue at first and it wouldnt let me sign in until i went to xbox.com... i had to agree to the new terms of service. Then it worked fine. Very nice!