MacLive is an Open Source utility to keep track of your Xbox Live friends on your Mac. You can set it to let you know when your friends log on via a Growl notification (so you can get fragging as soon as a buddy shows up online). Unfortunately, according this review MacLive is still a little rough around the edges, as the add a friend feature is apparently broken and there isn't yet a way to send messages, manage the friends list, etc.
Nonetheless, this is a great idea, particularly if the developer James Howard continues to improve it. MacLive is a free download at Google Code.