Friendz (Mac)
Friendz is an Xbox Live client. It's pretty good for seeing who's online and what they're playing. You can also send and receive messages, manage friend requests and view your achievements.
Growl handles all helpful notifications.
You can download it for free, bearing in mind you need Mac OS 10.5 or higher and an account at xbox.com.
There's also an iPhone version.
If you run into any problems, contact me.