Games are always more fun when you have someone to play them with, but if you're not always logged in to your Xbox LIVE account, how do you know when your friends are online? You can always log in and check, but where's the fun in that when you can hack together a traffic light to do it for you?
Andrew F hooked up an Arduino with an Ethernet shield to check every five minutes to see how many of his Xbox LIVE friends are online. For each friend, it records either a zero or a one, depending on their status.
He used this API from xboxleaders.com and Arduino's text finder library to find the online status, and you can probably guess what the colors mean. Andrew has his set up so that it lights up yellow for one friend and green for any number more than one, but you could easily change these.