Do you have an Xbox 360, do you have an IPod Touch or IPhone? If you do I would recommend downloading the Inside Xbox 360 app.
The Inside Xbox 360 app (which is the only app endorsed by Microsoft) allows you to keep track of you and your friends gamertags (up to 12 friends). You can also view community news, check out what the latest game and video downloads that are available on the marketplace, watch game trailers linked to YouTube and listen to latest Major Nelson podcast. There are other similar Xbox Live apps available, but I have found this one to be the most comprehensive and fastest. The Inside Xbox 360 app is also available for Blackberries and Windows Mobile Devices.