XBL friends list notifier app
Dennis from Nimblesoft has created a free app called 360 Friends that will tell you who is on/off Xbox Live, let you view gamercards, and notify you when a friend's status changes. Useful for determining who's having fun while you're stuck at work.
[Thanks DigitalGopher]
Update: I should have noted that 360Fanboy cannot vouch for the safety of this software. Please use caution.