Sometimes, with a full friend's list, loading it on a mobile device can be a chore. Enter the
Xbox Live Friend's Application.
The great part about this, in my eyes, is that you don't have to have your whole list on there. You manually enter the names, so you can add the people that you play with the most, and significantly cut down the size.
After adding the friends you want, it creates a link that you bookmark, and it will show you who is online.
The shots here are from the iPhone version, but it's available for Blackberry and Palm devices too.
Check them out.